As Floridians head to polls today, a likely dismal showing by former GOP frontrunner Rudy Giuliani will effectively end his campaign. In a rare moment of candor on Monday, the former New York mayor acknowledged as much, telling reporters "Wednesday morning, we'll make a decision." But while pollsters and pundits will attribute Giuliani's epic collapse to his cataclysmic decision to effectively skip Iowa and New Hampshire, his authoritarian arrogance or his 9/11 Tourette's Syndrome, Giuliani's fate was sealed during a decisive one week period in late November and early December.
That's what a quick inspection of the polls would reveal. Nationally, Giuliani's led the Republican pack with numbers that hovered between the high 20's and mid 30's until the end of November. Similarly, Rudy began his retreat from his New Hampshire (around 20%) and Iowa (mid teens) plateaus around the same time.
Those numbers and that timing would suggest that Giuliani was buffeted by two stories that broke almost simultaneously. First, Rudy suffered blowback from the bitter tone and nasty exchanges with Mitt Romney over immigration in the November 28 CNN/YouTube debate. Second and probably more important, Giuliani's untidy personal life and dubious professional ethics were laid bare by revelations that he used taxpayer funds to pay for covert Hamptons trysts with his then-mistress, now-wife Judith Nathan. Together, those two developments helped undermine Giuliani's aura of invincibility.
That his YouTube debate performance hut Giuliani was clear at the time. The Washington Times reported "Romney, Giuliani Get Testy," while the Washington Post emphasized the bickering that dominated the debate:

Former Massachusetts governor Mitt Romney and former New York mayor Rudolph W. Giuliani immediately set the tone for the combative event, using the first question to continue a weeks-long feud they have waged on the campaign trail. Each accused the other of ignoring laws against illegal immigration and distorting one another's record on the issue.

The timing couldn't have been more perfect for former Arkansas Governor Mike Huckabee. Coming just as his nascent campaign was beginning to take off, the debate catapulted Huckabee in the polls. The clear consensus that Huckabee's calm, civil tone won the debate may have been his campaign's tipping point.
Ultimately, though, the almost simultaneous story of the Judith Nathan taxpayer-funded romps (known in some quarters as the "Tryst Fund," the "Shag Fund" or "Sex on the City") was more damaging. It highlighted Rudy's serial marriages and messy personal life, all while crystallizing the picture of his questionable business practices. Ironically, the New York Times reported on December 20 that truth behind the story was less than met the eye, but by then it was too late.
At the end of the day, the man whose national career was jumpstarted on 9/11 saw it come tumbling down during 12/07.
UPDATE: With the Florida results now in, Rudy predictably produced his dismal third place showing. Time is reporting that Giuliani may drop out as soon as tomorrow and endorse John McCain. That report has since been confirmed by the AP.