DES MOINES — John Edwards willed himself through a 36-hour marathon, Senator Obama dispatched canvassers onto frigid streets and Senator Clinton hand-carried bagels and coffee to volunteers today, vying for victory in the Iowa caucuses and priceless momentum in the race for the White House. Leading Republicans exchanged routine unpleasantries on a final day of campaigning.

“You just don’t know what is going to happen,” said Mitt Romney, unwilling to forecast success over a Republican rival Michael Huckabee in tomorrow’s first contest of the race for the White House.

Increasingly, the candidates looked beyond Iowa to the states that quickly follow. Senator McCain spent most of the day in New Hampshire, which holds a primary on January 8, and his campaign ordered television advertising in Michigan, with a primary one week later.

But first there was Iowa, snow piled high and frozen — and an electorate warmed by the attention of Republican and Democratic hopefuls in the most wide-open presidential race in a half-century or more.

Late pre-caucus polls generally pointed toward a close three-way finish among Democrats and an unpredictable two-man struggle for the Republicans. A quarter of likely caucus-goers reported they either had not made up their minds or could still change them. That wasn’t a surprise in Iowa, where 21% of participants in the 2004 caucuses said they had made up their minds in the final three days.

That only added to the urgency of the campaigns, which stood ready with snow shovels and baby sitters — to make sure supporters were able to leave home for the caucuses — and delivered reminders to voters via Facebook and phone. Mr. Romney said his campaign made 12,000 calls on Sunday alone.

Unsurprisingly, there were reports of campaign dirty tricks — anonymous phone calls to Romney supporters directing them to incorrect caucus locations, and a recorded message disparaging a former senator, Fred Thompson of Tennessee, who hoped for a third-place finish to rescue his faltering candidacy.

There were predictions of a heavy Democratic turnout from election officials in scattered locations, who also reported independents switching their registration to Democratic so they could vote. “From what I can ascertain from the calls that we’re getting, it looks like the Democratic caucuses are just going to be flooded,” an elections supervisor in Scott County along the state’s eastern edge, Richard Bauer, said.

Mr. Obama, the most viable black presidential candidate in history, has bet his campaign on the support of first-time caucus goers, independents as well as Democrats who could be attracted to his message of political change. A victory in Iowa would validate the strategy, and presumably give him a boost in New Hampshire, where independents can vote in either primary.

At the first of five stops today, he brushed aside claims that he might be too inexperienced for the job. “The real gamble would be to have us do the same old things with the same old folks over and over and again,” he told a crowd of several hundred in Davenport, across the Mississippi river from his home state of Illinois.

Urging supporters out into the cold for more door-to-door campaigning, he said, “I hope you guys have your long johns on. It’s a little brisk outside. … Walk quick, talk fast.”

Mrs. Clinton, hoping to become the first female president, raced through five appearances, starting out by bringing bagels, fruit, and coffee to her volunteers in Des Moines. “I know if you’re here from Iowa to help me, this is like, nothing,” she said, referring to temperatures in the single digits. “But we have staff and volunteers who’ve come from exotic climes like California and they’re freezing.”

The former first lady ended her campaign with a two-minute advertisement broadcast statewide. “If you stand with me for one night, I will stand up for you every day as your president,” she said in the recorded remarks. “I’ll work my heart out to bring the country we love the new beginning it needs and I will be ready to start on Day One.”

Mr. Edwards, the party’s 2004 vice presidential nominee, was on a final campaign whirlwind. He raced through 12 stops during the day, at each one summoning Iowans to battle special interests in Washington by launching him on his way to the White House. He criticized the Bush administration for granting no-bid contracts to Halliburton and complained about tax breaks for the wealthy. “When will it stop? It will stop when we stop it,” he told a group in the Ivy Bake Shoppe in Fort Madison.

Later, at a supporter’s home in Mount Pleasant, he added, “When we rise up to take this country back for the middle class, we will be unstoppable.”

Mr. Romney, more than his Republican rivals, needed a first-place finish in Iowa, where he has outspent his foes by a wide margin in hopes of making himself the man to beat for the presidential nomination. A win would allow him to turn back Mr. Huckabee’s surprising ascent in Iowa, and give him bragging rights as he pivots to confront Mr. McCain in New Hampshire.

Like Mr. McCain, Mayor Giuliani was already in New Hampshire today.

Polls in Iowa show Mr. McCain making last-minute headway since he received an endorsement from the state’s largest newspaper, The Des Moines Register. A third-place finish within reach, he decided to fly back for one final round of Iowa campaigning.

Mr. Romney sounded almost nostalgic as he made his way through the day, but he found time to poke at both his leading rivals.

He defended himself against a McCain ad challenging his readiness to manage national security issues, saying he ran the Salt Lake Olympics after the terrorist attacks of September 11, 2001, and had drafted a state homeland security plan while governor of Massachusetts.

“Senator McCain is an honorable person, He’s been in the Senate for 25 years or more. And so people have a lot of talk there, a lot of suggestions about what other people ought to do, but I’ve actually been leading during that time.”

Mr. Huckabee, his ascent powered by evangelical Christian voters, told voters he was a consistent conservative — a not-so-gentle reminder that Romney once supported abortion rights and gun control laws. “You can look at my record and find out that all the way back, as far as you can find me saying or doing anything, I believe the sanctity of human life is a key, critical cornerstone issue for the future of our country,” he said in Fort Dodge.

In his latest unorthodox move in a campaign filled with them, Mr. Huckabee left the state on caucus eve to fly to California for an appearance on Jay Leno’s TV show.

Even that became grist for his critics.

“I guess he’s more focused on the caucus in L.A. rather than the caucus in Iowa,” Mr. Romney said.