DES MOINES, Iowa (AP) -- Throwing elbows to the end, Republican candidates appealed to Iowa residents who were casting the first votes of the 2012 presidential race on Tuesday for a strong send-off into the long campaign season ahead.

Mitt Romney, a confident-but-cautious front-runner for the GOP nomination, looked past his Republican rivals to President Barack Obama in his final pitch to voters.

"This has been a failed presidency," he told voters in a Des Moines ballroom. "I will go to work to get Americans back to work."

With large numbers of likely caucus-goers still undecided or willing to change their minds, the outcome in Iowa was uncertain right up to the finish line in a race that has elevated and then discarded a head-snapping assortment of front-runners.

The two who appeared most likely to challenge Romney for victory in Iowa were former Pennsylvania Sen. Rick Santorum and libertarian Rep. Ron Paul of Texas - neither of whom is likely to present as serious a challenge to Romney over the long haul as would former House speaker Newt Gingrich or Texas Gov. Rick Perry.

"It might come down to the speeches at the caucuses," said Phil Ubben, of Sioux City. "I want to support someone who can go all the way and defeat the Democrats in November."

The candidates pinned their final hopes on such voters.

"I think anybody can come in first," Gingrich said on CBS' "The Early Show." That was most likely wishful thinking for the former House speaker, who lost momentum after surging to the front of the GOP pack just weeks ago.

Training their sights on the pack leader, Gingrich, Santorum and other GOP rivals questioned Romney's conservative convictions and predicted Obama would, to use Gingrich's words, "tear him apart."

Santorum, appearing on ABC's "Good Morning America," said Iowans are "looking for the candidate they can trust, and that's why we're moving up in the polls."

On Tuesday night, Republicans will gather in living rooms, high school gymnasiums and local libraries for caucuses that start the process of picking the GOP nominee. In each precinct caucus, voters will urge their friends and neighbors to support a preferred candidate. For all of the attention paid to the caucuses, they are essentially a nonbinding straw poll that awards no delegates. Republicans do that at county and district conventions later in the year.

Twenty-five delegates are at stake in Iowa, out of 1,144 needed to win the Republican nomination - what Romney called "the whole enchilada."

Obama isn't ceding the stage to the Republicans while they sort that out: The president, fresh off a 10-day Hawaiian vacation, made plans to host an evening web chat with supporters in Iowa as the caucuses were under way.

For all the talk of trust and electability, candidates in both parties know the economy is sure to be the central issue this election year: Obama was traveling to Cleveland on Wednesday for an event focused on the economy. Romney, for his part, said he's running to get the country back on track after Obama's mistakes.

Most polls in recent days have put Romney and Paul atop the GOP field in Iowa, with Santorum in third and gaining ground. More than a third of all potential caucus-goers said they could yet change their minds. Perry, Gingrich and Minnesota Rep. Michele Bachmann all trailed.

Romney faces the same challenge he did in 2008: winning over a conservative base that's uncomfortable with his moderate past. In 2008, socially conservative voters united behind former Arkansas Gov. Mike Huckabee, denying Romney a first-place finish and contributing to his eventual defeat.

Romney's 2012 rivals worked to the end in Iowa to exploit questions about his conservative convictions. Perry, speaking to Fox News Channel on Tuesday, dismissed Romney as a "conservative of convenience." Bachmann offered herself as the "one true conservative."

Gingrich said Romney should "level with the American people" about his moderate political views. Asked on CBS if he was calling the former Massachusetts governor a liar, Gingrich said flatly, "Yes." Early on, Gingrich had insisted he'd run a positive campaign, but that strategy shifted after he sank in the polls under the weight of negative ads.

"Iowans have an opportunity to send a message to Washington and to the political system that the age of negative consultants and negative attack ads is over," he told voters in his closing pitch.

Emotions - and rhetoric - were hot in the campaign's final hours.

Santorum, blaming the Paul campaign for recorded phone calls questioning his policies on guns and abortion, told reporters at Fox News that "Ron Paul is disgusting."

Romney said he can handle any criticism his Republican rivals heap on him, calling it only a warm-up to whatever will come from Obama's camp. "My shoulders are wide," he insisted on Fox.

This time, Romney's trying to win Iowa by arguing he's the most electable candidate against Obama - a pitch that's winning over conservatives who desperately want to beat the president.

"I want to make sure I vote for and caucus for someone who is a winner. We cannot have another four years of Obama," said eyeglass salesman Paul Massey, 65.

How many people turn out to vote will help drive the results. In 2008, more than 120,000 Republicans showed up, a record. Weather could be a factor in this year's attendance. Iowa hasn't had much snow this winter, and there were clear but cold forecasts across the state.

After Tuesday's vote, Romney, Gingrich and Santorum planned to depart immediately for New Hampshire. Romney holds a commanding lead in polls there, and will be in a strong position to win even if he doesn't pull out a victory in Iowa. Paul plans to join his rivals in New Hampshire later in the week. The primary is Jan. 10.

Perry and Bachmann, both short-lived front-runners, don't plan to compete in New Hampshire, instead heading straight from Iowa to the first-in-the-South primary, set for Jan. 21 in South Carolina. Bachmann pronounced herself ready to move on: "This election is far from over," she said. "This is the opening chapter. Tonight is the first vote. We've got a long road to go."

Perry tried to buck up disheartened supporters by comparing caucus day to historic military campaigns of yore: "This is Concord," he said. "This is Omaha Beach."

Romney also plans to visit South Carolina this week, with campaign stops Thursday and Friday.

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