Opinion seems to be all over the lot on this question. And my first reaction is a Michael Bloomberg run will hurt the Democratic nominee, because at his core Bloomberg is a liberal and his maximum appeal will be to Democrats and Democrat-leaning independents. In the same way Ross Perot at his core was a conservative with an appeal that was clearly more to Republicans and Republican-leaning independents. At the end of the day, this simple analysis is probably the most accurate gauge of the likely impact of any Bloomberg candidacy.
However, if he does run I don't know that it would be as simple as that. Election 2008 is shaping up to be a bizarre and different election on so many different levels that I can begin to imagine scenarios where a Bloomberg run could hurt the Republican nominee more.
What makes this more intriguing is the likelihood of Bloomberg getting in is inversely related to the strength of the eventual major-party nominees. A Romney-Edwards general election would be Bloomberg's best hope and in the unlikely event they are both the nominees I think a Bloomberg run becomes a near certainty, with a Bloomberg presidency a possibility.
Of the other leading candidates Rudy Giuliani, Fred Thompson and John McCain for the Republicans and Barack Obama (and Al Gore) for the Democrats it is hard to envision the type of election where Bloomberg could find the opening to get the 35 to 40 percent of the national vote which is what he would need to win.
The best argument I heard that a Bloomberg candidacy hurts Republicans was from Frank Luntz on Hannity & Colmes:
COLMES: You said Bloomberg jumping in hurts Republicans more than it hurts Democrats, why?
LUNTZ: Because the Democratic base is stronger at this point than the Republican base. The Republican base has eroded over the last two or three years......Hillary Clinton has got a base vote of about 38 percent, 39 percent. She can't go any lower. The Republican base vote now is about one-third.
I think Luntz makes a very good point here: the Democratic base is stronger and more eager to win than the Republicans, but this is exactly what makes a Bloomberg run — as long as Hillary Clinton is the Democratic nominee — very unlikely.
Bloomberg's dilemma is this: even if Luntz is high with his 38 percent to 39 percent projection for Hillary's base vote and it's really more around 35 percent, that's still too high for Bloomberg when you consider that the Republican base vote is at least 30 percent. That leaves Bloomberg no pathway to victory.
Bloomberg needs the biggest possible opening, the kind that could be created by a progressive run by Edwards on the Democratic side and a less popular candidate on the Republican side that would give Bloomberg the opportunity to go after moderate, suburban voters turned off by an energetic embrace of social conservative issues. Romney-Edwards is the kind of scenario where Bloomberg could plausibly thread the needle in a three-way (33-33-33) type of race. Don't misunderstand me — it is still an uphill battle, but it is at least conceivable.
So, to answer the original question, a Bloomberg candidacy would hurt the Democratic nominee in most cases — except for the field that would generate the highest probability of Bloomberg actually getting into the race. In that scenario, Bloomberg could easily end up hurting the Republican as much, if not more, than the Democrat.