Is Spitzer not liberal enough? I can't think of any other reason (unless it's a matter of personal differences) that Schumer would jeopardize such a sure-fire candidate for the governor's seat.
Suozzi's actually staked out positions that are more conservative... for instance, he supports restrictions on late term abortions and has recently decried Spitzer's handling of "medicaid fraud," which is typically a GOP way of making it more difficult for poor folks to get health care...
Suozzi seems more suburb-focused, so his version of liberalism is probably a bit more bougeois than Spitzer's...
From what I've heard about Souzzi I still like him, but I find it troubling that he's so agressively considering this challenge, particularly when it involves getting support from those whom Spitzer prosecuted for defrauding investors...
As for Schumer, I think its sour grapes because he wanted to seek the Governor's office. If it devolves into a nasty and expensive primary, I'm going to be very unhappy to see Dem money go to such a futile endeavor, though, if Suozzi's getting GOP money, he can go ahead and waste it...
