How hard will it be for Republicans to oppose Hillary's legislation with a Democratic House and Senate? There really is no way we can retake either house in '08. So you're hedging all your bets on the 2010 elections swinging Republican, which may or may not happen as its impossible to tell what the political climate will be now.
In the meantime, how much legislation do you think Hillary could get passed in the two years before 2010, not to mention the likely judicial picks she'll get?
No harder than it was with 43 seats in the Senate to block Hillary Clinton's health care plan in 1993. Or stop Bill Clinton's proposal of gays in the military. Very little got done in the first two years of the Clinton administration.
What will be impossible is to stop any liberal legislation Giuliani might push for with what will likely be a shrinking minority of Republicans in the Senate.
And, if Hillary and the Dems push liberal legislation, it will pave the way for the GOP to retake Congress, probably in 2010, whereas they have no chance of doing so if Giuliani reaches office.