David Frum has an excellent rundown on why this would purely be a vanity piece for Bloomberg. (I agree)
More importantly though who could it hurt? You guessed it, the side of the angels.
So far, none of these winners and losers will affect the general outline of the election much. But here's one more prediction: Bloomberg will launch his campaign on the high road, giving worthy speeches about rebuilding US alliance, reforming health care, combating climate change, and other topics designed to win him the approbation of the Manhattan financial and media elite. Favorable publicity may bump him up to 5% or 8% in the polls, just enough to keep him spending money. But after the conventions of the summer of 2008, voters will begin returning to their homes in the two big parties. Bloomberg's numbers will dwindle (as Nader's did). He will then face a stark choice: accept that he's been made a monkey of - or up the ante. Nobody gets to be as rich as Bloomberg if he is not a fierce competitor. So - assuming he has followed the path thus far - he will double down. He will go negative, filling the airwaves with harsh attack ads.
Against whom will those ads be aimed? A lot will ride on that question. Attack ads are dangerous things, because they damage both the attacker and the attackee. Their main effect is not to change votes from D to R or R to D, but to depress turnout among potential supporters of the targeted candidate. Candidates refrain from excess negativity for fear of damaging their own image. But a Bloomberg in the polling basement will feel no such constraint.
The ads will be a free gift to the candidate Bloomberg dislikes less at the expense of the candidate he dislikes more.
And the candidate he dislikes more will almost certainly be the Republican.
Frum has got this bang on. Bloomberg has shown admirable initiative (schools, smoking, congestion tax) but is somewhat meddlesome (schools, smoking, congestion tax) and may have a serious tiff if Rudy Giuliani is the GOP nominee and they start to argue on who is the better mayor.
Hope he decides to save his $75-150 million.