SIOUX CITY, Iowa — The road to the White House has a speed limit of 70 miles per hour. It's a sprawling hunk of Interstate 80 that cuts through Iowa like a concrete plow, connecting Bettendorf, Des Moines, Guthrie Center and all stops in between. Its FM-radio soundtrack is a rough mix of classic rock, country and contemporary Christian; and truck stops and fast-food drive-thrus burn their logos into the black-velvet night. It's the Heartland, and if you're going to get to Washington, you've got to go through the Heartland.

It's not just about winning the hearts and minds of Middle America. It's very much about winning Iowa, the first state to anoint its choice for the two chief combatants for the White House. The Iowa caucus has marked the official start of presidential primary season for 25 years now, a badge it wears with pride. (You know that guy on the messageboard who posts "FIRST!"? That's Iowa.) And as other states shifted their '08 primaries around in hopes of stealing some of Iowa's thunder, the Hawkeye State fought back a few weeks ago, moving its caucus up to January 3, keeping a meaty grip on its "first in the nation" claim. Having won that battle, this is once again where underdogs can become big dogs, where frontrunners can become also-rans and where conventional wisdom can take a flying leap.

How important is Iowa? The Des Moines Register recently reported that more than three-fourths of caucus voters had been contacted by a presidential candidate, and one-fourth had personally met one. It's why candidates in both parties have already logged more than 500 days in Iowa this year. It's why John Edwards will roll through Sioux City, Carroll and Jefferson on Friday alone. It's why Barack Obama is in the midst of a five-day Change We Can Believe In Tour through Cedar Rapids, Bettendorf, Muscatine, Burlington, Fort Madison, Fairfield, Ottumwa, Chariton, Knoxville and Sioux City (we can only hope an enterprising bootlegger will be selling tour T-shirts in the parking lot). Yes, the next leader of the free world could well be chosen based on what happens at a middle school in **** Rapids.

Further proof of the import of Iowa comes on Saturday when everyone hoping to make a play for the White House on behalf of the Democratic Party will be in Des Moines for the most important event in the race for Iowa. It's called the Jefferson Jackson Dinner, a fundraising event for the Iowa Democratic Party. It happens every year, but things get a bit more interesting (and a bit more presidential) every four years. The candidates will have an audience with 9,000 Iowa Democrats, providing a chance for each to demonstrate they're the one worthy of the support, the resources and the pull of the Iowa Democratic Party.

And with polls showing Edwards, Obama and Hillary Clinton running neck-and-neck-and-neck in Iowa, it should make for an interesting afternoon. The three frontrunners will all speak at the dinner, as will fellow Democratic hopefuls Joe Biden, Bill Richardson and Chris Dodd, but the real show happens outside. Downtown Des Moines — which just a few weeks ago was occupied with Oktoberfest (shout-out to Barefoot Becky and the Ivanhoe Dutchmen, one of the finest polka outfits you'll ever see) — will be overrun with rallies, speeches and marches, all designed to flex some serious political muscle. Candidates are rallying their troops, calling in big-name reinforcements (expect a few Grammy and Oscar winners in the mix) and bracing for battle.

And that, of course, is why we're back in Iowa ... we'd never miss a good old-fashioned throwdown.

(Look for daily reports from Iowa as the candidates march towards Des Moines.)