Some pundits have speculated about whether former NYC Mayor Michael Bloomberg might be interested in running for President in 2016? 
It is hard to assess the likelihood of this happening. Bloomberg has been going around the country giving speeches on several topics, including gun control, the desirability of open primaries and the problems he sees when political parties do not pick centrist candidates.
What is not properly appreciated are the different scenarios under which a Bloomberg candidacy could happen; the paths that give Bloomberg credibility as a potential future President of the United States.
Make no mistake, each of these scenarios represents a potential "bad dream" for Hillary Clinton...some scarier than others!
The prospective candidate. Bloomberg is clearly a credible prospective candidate for President. He presents as the classic liberal-independent with successful managerial credentials from both inside and outside government. 
He is the potential maverick in the 2016 candidate mix, speaking his mind candidly and clearly.
His age could be a factor. He is 72 and would be a few weeks away from turning 75 on inauguration day 2017. 
Ambiguous or androgynous? One significant factor - Bloomberg's party affiliation has been enigmatic. Bloomberg is a Democrat who became a Republican to run successfully for Mayor of New York City. Subsequently he left the GOP to become an independent.
There are those who might reasonably ask if he is an opportunist or perhaps simply a committed political cross-dresser? Does he mean what he says or is he simply telling people what he thinks they want to hear?
Bloomberg's primary political appeal is to the center-left; centrist D's, independents and moderate and liberal R's. His uncompromising position on gun control, climate change and abortion have also made him a hero to many voters further to the left.
He is one of the wealthiest Americans and would be capable of self-funding his campaign should he wish to do so.
What would a Bloomberg for President in 2016 campaign look like? What would be his chances?
To begin to answer those questions, it helps to look at the recent past.
2012 and Americans Elect. It is worth noting that this is not the first time that former-Mayor Bloomberg has been seen as a prospective candidate for President. 
In 2012, there was an independent Internet-based effort aimed at creating a new center-left third party called Americans Elect.  This new group's well-funded agenda was similar to Bloomberg's. Its aim was to nominate a presidential ticket from what they regard as the political center.
The appeal of Americans Elect to any serious potential third party candidate was its ultimately successful effort to pre-clear ballot access for the new "party" so that their chosen candidates could start out the 2012 presidential campaign with their names appearing on the ballot in all fifty states.
This was both a significant and pragmatic achievement. Previously, all third party candidates, after having their moment in the sun when they announced, had to spend the entire presidential campaign (including most of their limited funds) fighting in the courts for ballot access (so that their candidates' names would not need to be written in). 
Many spoke of then NYC mayor Bloomberg as a dream candidate for Americans Elect; some even speculated that his supporters may have been behind it. 
His chances, however, seemed to depend on the R's nominating someone for President in 2012 who was seen as being from the party's right wing (e.g., former Alaska Governor Sarah Palin) so as to create the large group of potential swing voters in the middle necessary to attract a plurality of American voters in enough states to win the electoral college.
Instead, the R's nominated former MA Governor Mitt Romney, whose political credentials and pedigree were more towards the middle.
Bloomberg eventually made it clear he was not interested (he endorsed President Obama) and Americans Elect decided not to nominate a presidential and vice presidential candidate to contest the 2012 election. 
2012 was not Michael Bloomberg's year. 2016, however, may be another matter.
There are four possible scenarios under which Bloomberg might run in 2016, some of which have significant potential.
Scenario 1: While unlikely, running for the Republican presidential nomination is at least a theoretical possibility for Bloomberg (and interesting because as the Republican nominee, Bloomberg would arguably have a chance of carrying California).
A Bloomberg Republican candidacy is unlikely but still worth considering. Bloomberg did serve for two terms as the Republican Mayor of New York City, even though he subsequently left the party to run for his third term as an independent. 
Bloomberg's appeal to independents and disaffected moderate and liberal Republicans could make him a viable R candidate if nominated. No R nominee since George H. W. Bush in 1988 has been able to carry California and that state has served as the D's Presidential electoral base in the modern era. Bloomberg running on the R ticket could conceivably put the state in play in the same way a Republican Arnold Schwarznegger did when he successfully ran for Governor.
The simple fact, however, is that Bloomberg has little or no chance of winning the Republican nomination. He left the Republican party where he was never that comfortable. Many Republicans have not been comfortable with him, considering Bloomberg a RINO (Republican in Name Only) and an opportunist.
It would take a candidate with much greater mass popular appeal (like Eisenhower in 1952) to overwhelm the preferences of regular Republicans on this question. In that respect, Bloomberg does not have anything close to former Governor Schwarzenegger's star power.
Additionally, even if successful, his actual chances of making California competitive on the Presidential level against a D candidate like Hillary Clinton could not be guaranteed.
Consequently, it seems that any effort on his part to get the 2016 R nomination would be doomed to failure.
That said, an unsuccessful campaign for the Republican nomination might pave the way for a credible third party run, similar to that made by former Congressman John Anderson after he went after the R presidential nomination in 1980.
Scenario 2: Most people thinking about Bloomberg as a prospective 2016 candidate have him on the Third Party path.
Americans Elect is still there as an organization and movement. Although a good deal of their earlier work might need to be repeated, it would likely be easier to replicate their earlier success the second time around. 
Americans Elect once again would seem to be a tailor-made vehicle for Bloomberg, whether that came about through a failed run as a Republican or (as would seem more likely) as an independent candidacy from the start.
What is clear is that a three way race would scramble everyone's electoral vote calculations. Blue states like California and New York could ultimately end up in Bloomberg's column or they could stay with the D's....or they could end up split between Hillary and Bloomberg, putting one or more of those states in the Republican electoral vote column by a plurality. 
As in 2012, Bloomberg's chances of winning a three way race in 2016 would depend on the size of the gap separating the candidates of the two main parties.
For the most part, to Bloomberg's left, Hillary Clinton presents less of a target than President Obama did in 2012; She is not as far to the left as President Obama and, as such, would leave slightly less contestable room.
As in 2012, the farther the R's go toward the Tea Party or the Rand Paul libertarians in selecting their nominees, the more opportunity there would be for Bloomberg running as an independent.
To the extent Bloomberg was successful, one possible outcome would be to put the election into the House of Representatives (where the Republicans would be expected to have the advantage). 
Bloomberg would need to be overwhelmingly successful (in a way that no third party candidate in the modern era has ever been) and win the electoral college outright with a plurality of the popular vote not to have that be the result.
Interestingly, last May on Face the Nation, Bloomberg indicated he did not think it was possible for a third party candidate to win the Presidency. While he indicated he would be interested in running for President if he thought he could win, he indicated was not interested in a third party run in 2016.
"(N)o matter how much the press wants to create an Independent because it's good for seeing newspapers and inches and minutes....it's just not possible." 
It doesn't sound like he plans to change his mind about this, which makes it another unlikely scenario.
Scenario 3: Clearly, the most intriguing 2016 scenario would involve Bloomberg going after the Democratic presidential nomination.
It is surprising that this scenario hasn't been more developed in the media, but it is clearly the one that has the greatest potential and the one that makes the most sense.
It is also where the nightmare gets the scariest for the Clintons.....the possibility of someone unexpected who would credibly challenge Hillary Clinton for the Democratic nomination she already feels is hers.
It's not like it hasn't happened before....
It has been noteworthy that few D's have been willing to challenge Hillary Clinton for the nomination in 2016. This has been attributed to the Clinton's history of punishing those who cross them. 
At the moment, there are others who look like they are willing to run for the D 2016 nomination, but none of these potential candidates currently seem to have the potential to mount a successful challenge.
To Hillary Clinton's left, Maryland Governor Mavin O'Malley seems to be running but is a relative unknown outside Maryland.  So is VT Senator Bernie Sanders, who currently, like Bloomberg, is officially listed as an independent.  The Kennedys have reportedly encouraged Sen. Elizabeth Warren to run. Warren, however, has indicated that she is not interested. 
All three of these candidates would be coming at Hillary from the left, which is where most people see an opening.
Bloomberg would be a threat to Hillary, however, from all sides; from right (as a former R and proven manager), left (gun control and as a home for Obama people who don't want another Clinton presidency) and center (as someone who appeals to independents).
In short, it represents the kind of potential to surround that is the opposite of "triangulation."
It would be a major fight, but one worth making if Bloomberg wants to be President.
Bloomberg could declare his return to the Democratic party tomorrow if he wanted to. There is nothing stopping him. Such a declaration would immediately start speculation along these lines (and begin the Clinton attempts to discredit him).
In fact, he really doesn't need to make this type of announcement for "draft Bloomberg" efforts to spring up around the country and in states like Iowa and New Hampshire.
Clinton efforts to head this off would likely only feed any fires sparked by such efforts....making them appear more like spontaneous combustion.
The worst part of this scenario for Clinton, even if she wins the nomination, would be having to spend several months attacking the prospective champion of the very voters she would need to win in a race against the Republicans in the Fall.
It's not just the D's who supported Bloomberg she would have to worry about coming home, but the independents and disaffected R's that Bloomberg appeals to and who would constitute a substantial portion of the "swing voters" she would need to prevail.
It is noteworthy that "Party switcher" is not a political epithet any more! Once changing parties was seen as the kiss of death. Recall Winston Churchill's famous line about switching from the Conservative party to the Liberal party....and then back again.
Anyone can "rat," but it takes a certain amount of ingenuity to "re-rat." 
As an indication of how much this has changed, today's political parties are now constantly on the look-out for incumbents willing to consider switching parties and joining their ranks. In the current election alone, the former R Governor of Florida, Charlie Crist, is running for election to the same job as a Democrat. 
Ask them why they switched and they will probably echo Ronald Reagan's line about his switch from FDR-Truman Democrat to an Eisenhower-Nixon Republican.
"I didn't leave the Democratic party....the party left me!" 
Bloomberg also has been active with groups supporting D's in 2014 mid-terms, such as Emily's List. He recently donated $2 million to a group called "Blue Senate 2014" aimed at helping D's maintain their Senate majority. 
A Bloomberg announcement that he was seeking the D nomination would immediately throw everyone's 2016 political calculations up in the air.
There would likely be an immediate effort by the Clinton camp to de-legitimize Bloomberg as a D, but that would not be likely to survive the first set of polls.
There is no shortage of people in the Democratic party who would like someone other than the Clinton's to back and they would expect even to find some sympathy in the White House. 
The potential to re-live the 2008 primary season with a series of close primary elections and caucus outcomes, each allocating a proportional share of votes to the candidates, would make for a prolonged battle and a good deal of trench warfare.
It will mean that additional candidates on the left like O'Malley and Sanders would have an incentive to continue their campaigns right up to the convention with the prospect of being the difference that forces the main candidate to come to terms in order to clinch the nomination.
The problem with this approach is that it is the kind of challenge that has the potential to tear the party apart; to end up making the nomination not worth having. 
Which D candidate President Obama eventually supports will be a factor. It is unlikely that President Obama would get involved officially until the convention, however, and only if it would make a difference. He is more likely to remain officially neutral above the fray and be yet another factor that the two main candidates will need to reckon with.
That said, it's not hard to envision who among these two contenders Barack Obama might expect to have a better relationship with as a new former President. Bloomberg has consistently had better relations with President Obama than the Clintons, whose desire has reportedly been to treat Obama's presidency as an 8 year detour away from an otherwise inevitable Clinton succession. 
If Bloomberg succeeds and gets the D nomination, he would be seen by many as on the path to the White House. As with Hillary, however, it would not be a lock on the outcome. Much would depend on who the Republican's nominate. Then there is also the various New York issues (regulating the size of soft drinks that can be sold at convenience stores?) that you just don't know how they will play outside "Big Applelis." 
Losing the D nomination would seem like the ultimate nightmare for Hillary Clinton,....but there is yet another scenario that could arise even if she becomes the D nominee.
Scenario 4: If Bloomberg does not get the Deomcratic nomination....the third party path would likely still be available (creating an incentive for the D's putting him in the Vice Presidential slot).
It really depends on how Hillary would win the D nomination for things to get to this point. If Bloomberg felt he had been denied the nomination through trickery, fraud or perceived illegality (as Teddy Roosevelt and his supporters did in 1912), you could end up with a break-away Bloomberg faction demanding that their cause be taken directly to the voters in November.
Bloomberg also might simply be uninterested in being anyone's "number two." In the past, acrimonious conventions have come together by putting the losing candidate on the ballot as the running mate or with the passage of time (of which there is usually never enough).
This would take you to a variation of the third party run (scenario 2 above) with all the attendant electoral uncertainties and disadvantages.
At that point, however, it is usually no longer about winning....but about payback and righting perceived wrongs.
All of these scenarios are possible...it's just that some are more realistic than others.
It will be interesting to see if Michael Bloomberg does make a run for President....and, if so, just how he does it.
Clearly, the better choice would be for him to make a run for the D nomination.
For Hillary Clinton, it will be one of the supreme tests of her political skills to keep any such potential nightmare from becoming a reality.
8. See http://abcnews.go.com/blogs/politics/2012/05/americans-elect-ends-online-primary-after-no-candidates-qualify-to-run/