LE MARS, Iowa - A confident Mitt Romney looked to seize the momentum in the Republican presidential race with a victory in Tuesday's leadoff Iowa caucuses, but his rivals worked to undermine the perception that he was the candidate best positioned to defeat President Barack Obama.

The final Des Moines Register poll showed the former Massachusetts governor and Texas Rep. Ron Paul locked in a close race in Iowa, with former Pennsylvania Sen. Rick Santorum rising swiftly to challenge them. Nearly half of likely Iowa caucus-goers view Romney as the Republican most likely to win the November general election. He was far ahead of Santorum and Paul in this regard.

With Romney in a position of strength in Iowa, both Santorum and Paul went directly at Romney's chief argument -- that he is the most electable Republican in a head-to-head matchup against Obama next fall. They hope they can sway the roughly half of likely caucus-goers who say they are undecided or willing to change their minds two days before Iowa begins the state-by-state nominating contests spread over the next six months.

Only three or four candidates typically make it out of Iowa with enough momentum and money to continue in the race.

Paul and Santorum were fighting against the notion in Republican circles that their bases of support are narrow and neither would be able to cobble together the diverse voting coalition necessary to beat Obama in November. The libertarian-leaning Paul attracts legions of backers who like his message of states' rights and limited central government, while Santorum -- an anti-abortion crusader -- is popular among Christian conservatives who make up a large segment of the Republican Party base.

In contrast, Romney has styled himself as a Republican able to attract a broad spectrum of voters. As polls showed him with a narrow lead in Iowa in the past week, he has redoubled his effort to portray himself as the business-savvy executive with national appeal who is best able to challenge Obama on the campaign's most pressing issue, the economy. Polls show Obama is vulnerable as he seeks a second term, weighed down with voter dissatisfaction over the sputtering recovery from the Great Recession.

Although the race remains fluid, it appeared that Romney's carefully crafted plan to avoid underperforming in Iowa, where he campaigned little until last week, may be working given that conservatives have yet to coalesce around a single candidate. Iowa's large evangelical bloc is splitting support among Santorum, former House Speaker Newt Gingrich, Texas Gov. Rick Perry and Minnesota Rep. Michele Bachmann.

The issue of what type of candidate to choose cuts to the heart of why the Iowa race is so volatile; an NBC/Marist poll last week showed nearly even percentages of Iowa caucus-goers want a candidate who shares their values as want a candidate who can beat Obama.

"The first thing you see when you talk to any Iowa Republican is that desire to beat Barack Obama," Iowa Republican Chairman Matt Strawn said.

Mindful of that, both Romney and Santorum campaigned in Iowa on Sunday to make the electability case.

Romney was spending the final days of the campaign in areas he won four years ago when he finished second in the caucuses, rather than risk underperforming in places where more ardent conservatives are leery of his Mormon faith and shifting positions on social issues such as abortion and gay rights.

He is increasingly projecting confidence that he would be the Republican nominee, promising to return to Iowa, a general-election swing state, in the fall campaign.

On Sunday, Romney criticized Santorum directly for the first time since his opponent's rise -- though carefully -- saying the ex-senator "has spent his career in the government in Washington."

Santorum, who has spent more time in Iowa than any of his rivals, was campaigning in rural northwest Iowa, with stops in conservative counties won by former Arkansas Gov. Mike Huckabee, a Baptist minister, who finished first in the 2008 caucuses.

As he met voters, his final ad for Iowa TV called him "a full spectrum conservative" who is most likely to beat Obama and the "trusted conservative who gives us the best chance to take back America."

Paul, who Romney has said is outside the Republican mainstream, countered the suggestion that he's a fringe candidate. In an interview with ABC television from his home state, where he was spending the weekend, Paul insisted: "I'm electable. I've been elected 12 times in Texas, when people get to know me."

Paul, who has slipped somewhat in the wake of attacks on his foreign policy positions, argued in television interviews that the majority of Americans are with him when it comes to a non-interventionist foreign policy.

Along with Romney, Perry, Gingrich and Bachmann -- all of whom are trailing in polls and fighting for the support of Christian evangelicals -- spent the morning in church.

Gingrich went after Romney with abandon, saying he felt like he'd been "Romney-boated" and adding that the multimillionaire would "buy the election if he could."

The attack was a reference to a 2004 TV ad campaign by a group called the "Swiftboat Veterans for Truth" that bloodied Democratic presidential candidate John Kerry. This year, Gingrich has faced an onslaught of negative TV advertisements by a group aligned with Romney.

In all, at least $12.5 million in advertising -- much of it negative -- has flooded the Iowa airwaves in the run-up to the caucuses as candidates and outside groups aligned with them worked to influence the outcome of what has been a remarkably fluid and unpredictable campaign.

Working to make up ground quickly, Bachmann and Perry tried to make the electability argument while assailing Santorum, who after surging in the Iowa polls suddenly has found himself the target of sharp attacks from rivals.

Perry said on "Fox News Sunday" that his rival has "a spending problem," supporting pet projects in his home state and repeatedly voting to raise the debt ceiling while serving in Congress.

Former Utah Gov. Jon Huntsman, who served as Obama's ambassador to China, is skipping the Iowa contest to focus on New Hampshire, where Romney is heavily favoured in the first-in-the-nation primary on Jan. 10.