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.