OK, I'm starting to see a way that Giuliani's front-runner status might end. Taegan Goddard reports (emphasis added):

American Research Group is out with a new batch of presidential polls from Iowa, New Hampshire, Texas and Arkansas.

...Among Republicans, Rudy Giuliani and John McCain are tied in Iowa, McCain leads in New Hampshire, Giuliani leads in Texas and Mike Huckabee leads in Arkansas. Adding Fred Thompson's name to the ballot has hurt Giuliani in Iowa and New Hampshire.

So Thompson might be somewhat of a Giuliani-slayer. And then I don't know what happens if this guy gets going:

Having raised over $1 million for his presidential exploratory committtee, Rep. Tom Tancredo (R-CO) said it is far more likely that "he will follow through with a full-fledged candidacy" than run for Senate, reports the Rocky Mountain News....

Forget the actual positions these guys take on various issues -- both Thompson and Tancredo (Gingrich too) are seen as people who'll defend us against "them" -- just like Giuliani. Democrats? Terrorists? Immigrants? Serial killers stalking the Upper East Side? All these guys have impeccable "them"-fighting credentials (namely, they all show up in the media telling us they are "them's" sworn enemy).

Get enough of these guys in and it's a horse race again.

Thompson is actually beating Hillary Clinton by one point in a general-election matchup, according to a new poll -- but it's a Rasmussen poll, so I wouldn't make too much of it. (Rasmussen polls are highly skewed toward the GOP -- Rasmussen has Bush's job approval at 43%, for Pete's sake.)

I wonder if McCain could actually eke out a win as a result of all this -- all these other guys will split the "let's kick some ass" vote and he'll keep the votes of somewhat less rabid Republicans. That may not be how you see the GOP electorate dividing, but those are the lines along which it divides itself, as far as I can tell.