Despite the decisions of the Giuliani and McCain campaigns not to participate in the Ames Straw Poll in Iowa - a non-binding one-day mini-convention which serves primarily as a fundraiser for the Iowa GOP - Iowa may still be the key to the nomination in both parties, according to NBC News Political Director Chuck Todd:

I believe Iowa will matter a great deal. It has the CHANCE to END the campaign.

For instance, if either Giuliani or Clinton win their respective caucuses, they will have essentially won the nomination. If Obama or Edwards can't beat Clinton in Iowa, they will have a hard time beating her anywhere. Ditto for Mitt Romney, John McCain and Fred Thompson in their attempts to stop Giuliani. Iowa is easily the toughest state for both Clinton and Giuliani to win, so if they can make it there...

Read his entire article at the above link. He also argues the importance of New Hampshire is in doubt, but that if SC moves its primary up, that and Florida may signal the effective end of the contests before the massive cluster of primaries on February 5th (which would no doubt irritate all those states who moved up to 02/05/08 in order to "increase their influence on the nominations").

I'm not sure he's correct about Iowa being the easiest state to derail Hillary and/or Rudy, though. It is extremely competitive, and has been. Edwards and Romney have invested heavily in the state and no one should be shocked if they are the winners next January.

McCain has a strong following in New Hampshire, and strong appeal in veteran-rich South Carolina and aging Florida. Romney was neighboring Massachusetts' popular Governor, so he could get as much "home town" support as Giuliani might in the Granite State. Thompson is showing surprising strength in South Carolina without even announcing (the appeal of a candidate without an accent is strong).

South Carolina is Edwards' birthplace, and he won the primary in '04. Obama's status as the first major minority candidate (with some realistic chance to win) could appeal to the black voters who make up roughly HALF of the Democratic Primary electorate.

So, there will be other chances to derail the frontrunners. But Todd is correct that if Rudy and/or Hillary wins Iowa, it will make it much harder on their opponents. Hillary is the best situated to lose Iowa, with her successful fundraising continuing. Edwards may have to sink his whole bank account into the state to pull out a win, leaving him without much time to raise money and advertise in NH, SC, and Florida.