DES MOINES, Iowa — A yearlong prelude to the presidential election drew to a frenzied close here as Iowans on Wednesday absorbed a final torrent of campaign arguments before the state's caucuses formally open the race for the White House.

Sen. Hillary Rodham Clinton of New York asked voters at rallies and in a two-minute television commercial broadcast statewide, "Who is ready to be president?" Sen. Barack Obama of Illinois implored supporters to

believe in his candidacy, asking: "Who can take us in a fundamentally new direction?" John Edwards, a former North Carolina senator, pledged to represent struggling Americans, saying: "Who's going to fight for you?"

On the eve of the Iowa caucuses, only a portion of the Republican field spent the day courting voters in Iowa. Mike Huckabee, the former governor of Arkansas, flew to California to appear on "The Tonight Show," drawing harsh criticism from a leading rival in Iowa, Mitt Romney, the former governor of Massachusetts. Seeing a potential opening, Sen. John McCain of Arizona hastily flew to Iowa for a last-minute plea for support as he sought capitalize on the turmoil of the unsettled field.

The costliest campaign in the three-decade history of the Iowa caucuses headed to an unpredictable finish, as thousands of volunteers and campaign aides from across the country descended on neighborhoods and towns to coax voters to 1,781 precinct sites in the state on Thursday evening. Politics dominated the radio and television airwaves, with advertisements airing back-to-back from morning until night.

"Just one more day of commercials — until the general election," Obama told an audience in Davenport, asking for a final dose of patience from voters.

The most-sophisticated presidential campaigns that have ever been waged in Iowa — fully engaged for much of the year — ended in a flurry of old-fashioned get-out-the-vote efforts. The Clinton campaign, for example, has enlisted 5,000 drivers to ferry voters to the caucuses, particularly elderly women, who form a critical well of support.

The disparity in intensity between the Republican and Democratic contests was palpable in the final round of campaigning, with Romney, McCain and Fred Thompson, a former Tennessee senator, holding a series of small sessions with voters, while six Democratic candidates drew hundreds of people to boisterous rallies across the state.

Still, the outcomes for both parties depended on many variables, particularly in the Democratic contest, where Sens. Joseph R. Biden Jr. of Delaware and Christopher J. Dodd of Connecticut, and Gov. Bill Richardson of New Mexico staged vigorous campaigns. Candidates who do not reach a threshold of 15 percent in each precinct are deemed non-viable, freeing their supporters to align with a more-popular candidate.

The Republican contest has no such rules, but, for Democrats, the realignment of supporters could tip the balance of the contest, so the leading campaigns intensified efforts on Wednesday to identify voters' alternative selections. Those decisions also often hinge on local politics, adding a fresh dose of uncertainty to a volatile caucus night.

On the final day of campaigning, Obama's brigade worked to solidify support from independent men, moderate Republican women and younger voters — most of whom have never caucused before — to address concerns about the depth of his support in rural Iowa, which has disproportionate sway in the final outcome. He worked his way through four stops in eastern Iowa before holding a nighttime rally in Des Moines, with each event designed to be a meeting place for supporters to pick up materials for a door-to-door canvassing effort.

Clinton's team, meanwhile, dispatched 625 people to knock on the door of every voter who has signaled support and every undecided voter leaning her way. Her advisers lowered expectations for Thursday night, saying there would be no winners or losers if the three leading candidates finished closely bunched together.

After Clinton spoke in Cedar Rapids on Wednesday afternoon, her best-known supporter in Iowa, the former governor, Tom Vilsack, a co-chairman of her campaign, said he would not predict the outcome. But Vilsack also asserted that while he did not know whether Clinton would "win," he had no doubt that she had already "succeeded" in Iowa.

"When she got into this in March, she wasn't competitive here — she wasn't competitive," Vilsack said, putting it as plainly as he ever has that Democrats here were skeptical of a Clinton candidacy early on, leading a deputy campaign manager to suggest last spring that Clinton skip Iowa altogether.

Clinton was running statistically even with Obama and Edwards in some early polls, although she was quite a bit behind them in others.

Edwards waged a 36-hour marathon campaign swing, making 16 stops from the Missouri River in the west to the Mississippi River in the east. He also looked beyond Iowa by releasing an advertisement to begin running Wednesday in New Hampshire, which holds its primary on Tuesday, pledging to fight against corporate greed that has "infiltrated everything that's happening in this democracy."

Along the way, Edwards showed signs that he was more concerned with competition from Obama than from Clinton. Asked directly by a voter whether he could beat Obama, Edwards said, "Oh, yeah, I can beat him." Both men are engaged in a spirited fight to win over second-choice voters.

If the Democratic race remained unpredictable, the outlook for the Republican contest was also murky. With a captive audience in Iowa, Huckabee's last-minute trip to California added to the mystery behind his campaign strategy, but he said his appearance with Jay Leno would reach more voters than a day on the campaign trail.

Before he boarded his plane in Iowa, however, Huckabee announced his support for the striking television writers union and said he identified with the striking workers because he is an author. He said he was unaware that he would be crossing picket lines because of the strike.

Huckabee's inconsistency or ignorance about the picket lines outside the Leno show are the latest in a string of missteps that have underscored the ad-hoc, on-the-fly nature of his insurgent campaign.

Speaking to reporters in Bettendorf, Romney ridiculed his rival's out-of-state trip on the eve of the caucuses. "Frankly, my focus is on the caucusers here in Iowa," he said. "I think Mike is more concerned about the caucus in Los Angeles."

While the Iowa caucuses take place five days before the New Hampshire primary, Romney opened his news conference attacking his main rival in New Hampshire, McCain, rather than Huckabee, who is his leading foe in Iowa.

"Welcome to Iowa, Senator," said Romney, who criticized him for sponsoring the immigration reform bill in the Senate that would have offered a pathway to citizenship for illegal immigrants provided they cleared certain hurdles. His stance on immigration was among the reasons McCain limited his campaigning in Iowa, focusing instead on New Hampshire.

For all the back-and-forth among those candidates, though, perhaps the biggest uncertainty in the Republican field rests in the candidacy of Rep. Ron Paul of Texas, who has raised $19 million. On city billboards and on signs planted along roadsides and in fields across Iowa, Paul's visibility has soared in recent weeks, raising worries among his Republican rivals about the strength of his movement.