DES MOINES, Iowa — Senator Obama, Senator Clinton, and John Edwards fought for first today in Iowa’s presidential caucuses, a multimillion-dollar exercise in grass-roots democracy and the initial, critical test in the campaign for the party’s 2008 nomination. Mitt Romney and Michael Huckabee vied for the Republican victory.

Iowans rendered their judgments in meetings at 1,781 precincts from Adel to Zingle, in schools, firehouses, and community centers where the candidates themselves could not follow.

Earlier, the contestants had their final say.

Mr. Huckabee told a crowd in Burlington, “It’s about believing in a cause,” a summation that rivals in both parties could easily embrace. Likewise Mr. Edwards’s plea to his backers in Iowa City: “This thing could be really close. … We need you to make calls, talk to your friends.” And Obama in Des Moines, when asked how he was feeling: “I feel great, although my throat. …”

Win or lose, there was little time for rest. New Hampshire’s first-in-the nation primary is set for next Tuesday, and the campaign quickly accelerates into a rush of contests culminating in more than two dozen on February 5.

With President Bush constitutionally barred from seeking re-election, both parties had wide-open, costly campaigns.

Mr. Obama, a first-term Illinois senator, stressed a need for change as he campaigned to become the first black president in history. Mrs. Clinton boasted of her experience as she worked to follow her husband into the White House and become the first woman to occupy the Oval Office. Mr. Edwards, his party’s 2004 vice presidential candidate, cast himself as the implacable enemy of special interests as he aimed to improve on last time’s second-place showing in the state.

Governor Richardson of New Mexico, Senator Dodd of Connecticut, Senator Biden of Delaware, and Rep. Dennis Kucinich of Ohio also contested the state.

Mr. Romney, a former Massachusetts governor, stressed his background as a businessman and organizer of the 2002 Olympics, and he worked to persuade conservatives to ignore his earlier positions on abortion and gay rights. He ran the only commercials of the campaign critical of a rival, hitting Mr. Huckabee for his positions on immigration and the pardons he issued while governor of Arkansas.

Mr. Huckabee, an ordained Baptist minister, pinned his hopes on evangelical conservatives who accounted for 40% of caucus-goers or more. By his account, he risked his candidacy when he decided not to air commercials criticizing Mr. Romney.

Senator McCain, Rep. Ron Paul of Texas, and a former Tennessee senator, Fred Thompson, were also on the ballot, although their aides made no claim they were in the running for a first-place finish. So, too, for Mayor Giuliani, the former New York mayor who largely abandoned the state in the campaign’s final days.

Iowa sends 45 delegates to the Democratic National Convention next summer in Denver and 37 to the GOP gathering in St. Paul, Minn. But that was hardly the reason the crowded field of presidential hopefuls devoted weeks of campaigning, built muscular campaign organizations and spent millions of dollars on television advertising in the state.

For three decades, Iowa’s caucuses have drawn presidential hopefuls eager to make a strong first impression, and this year was no different.

Mr. Obama, Mrs. Clinton, and Mr. Edwards spent at least $19 million on television advertising among them, and all three capped their campaigns with statewide broadcasts yesterday. Mr. Romney told supporters in a final daylong swing around the state he had been in 68 of 99 counties since he began his quest for the White House, had spent 55 days in Iowa and spoken before 248 separate audiences.