He will be to Hillary Clinton as Ross Perot was to Bill Clinton....her ticket to the White House
Think! He's a New York Liberal, from whom is he most likely to siphon votes? Sure, he'll try to pass himself off as a "moderate", so will Hillary, but it won't wash in either case. If he took enough votes away from her Heinous in few New England states, it could represent a GOP margin of victory. Maybe that's why he's doing this, to pay back the GOP for nominating him for Mayor (or at least that's what Hillary may say).
The real "Ross Perot" threat is that "moderate" organization dreamed up by (among others) Jimmy Carter adviser, Ham Jordan. It's tailor made for the next disgruntled Republican who fails to secure the GOP Presidential nomination (McCain perhaps?). If that happens, the Republicans may thank their lucky stars for a Bloomberg "left jab" at Hillary. In any case, Mikey Bloomberg is grossly overestimating his personal charm.