Republican White House hopefuls made last-ditch pitches to woo voters across Iowa on Tuesday, with polls giving at least three - Mitt Romney, Rick Santorum and Ron Paul - a shot at winning the first contest of the 2012 presidential campaign.

Iowa's quirky caucuses are known more for weeding out candidates than picking the future president, and finishing in one of the top spots could provide a big boost to any contender in the volatile contest to pick a challenger to Democratic President Barack Obama in the November 6 election.

The tight Republican race, marked by rollercoaster ups and downs in opinion polls for most of the candidates, has sparked weeks of negative campaigning in the Iowa competition. Millions of dollars have been spent by outside Super PAC fundraising groups.

Former House of Representatives Speaker Newt Gingrich, a one-time front-runner who has been pummelled by a series of attack advertisements from one such group, lashed out at Romney on Tuesday for trying to distance himself from the ads.

Asked if he was calling Romney a liar, Gingrich said, Yes on CBS's The Early Show.

This is a man whose staff created the PAC, his millionaire friends fund the PAC, he pretends he has nothing to do with the PAC. It's baloney, said Gingrich.

Polls give Romney, a former Massachusetts governor, a narrow lead over social conservative Santorum and libertarian Paul ahead of Tuesday evening's caucuses. But many voters are still undecided, leaving the contest up for grabs.

I'm undecided, and I'm still in the same boat as when I came, said Zander Morales, a hospital worker in Des Moines after he attended a rally on Monday for Paul, a U.S. congressman from Texas. I'm not sure what I'm going to do.

The stakes are high for each of the six candidates competing in Iowa. Romney is aiming for a win that could ease persistent doubts among conservatives about his record and propel him toward clinching the nomination early.

Struggling rivals like Texas Governor Rick Perry and U.S. Representative Michele Bachmann are fighting for at least a fourth-place finish that could preserve their flickering hopes.

Gingrich, who led the race just weeks ago, aims to end his slide and prove he can make another comeback.

Santorum was trying to consolidate fractured religious and social conservatives in Iowa and emerge as the leading conservative alternative to Romney.

Romney's Iowa campaign chairman, Brian Kennedy, said he thought a higher turnout would bode well for Romney because he draws support from a broad swath of the party.

We're in a strong position but the unique thing about caucuses is you never really know, Kennedy said. If he does well, it will give him a lot of momentum going to New Hampshire, South Carolina and Florida.

FRENZIED SCHEDULE

The caucuses start a frenzied schedule for the Republican presidential hopefuls that will include a half-dozen debates in January and three more state votes -- on January 10 in New Hampshire, January 21 in South Carolina and January 31 in Florida.

Gingrich dropped in Iowa polls after attacks from Paul and the Super PAC backing Romney. Gingrich's jab at Romney in the intense contest's last hours was a sharp departure from his earlier promises to stick to a positive message.

More than 100,000 voters will gather across the midwestern state at more than 800 schools, libraries and other public spots starting at 7 p.m. CST (0100 GMT) to render judgment in a contest that could provide momentum and new campaign donations.

Results should begin coming in within a few hours.

The weather was expected to be fairly cold, but dry, which should boost turnout. Ice or snow would prompt some voters to avoid the evening meetings, which can last for hours.

Iowa's nominating contest has a spotty track record of picking winners, but has traditionally cleared the field of losers and elevated surprise contenders. Former Arkansas Governor Mike Huckabee won the 2008 Republican caucuses, and the eventual nominee, Arizona Senator John McCain, finished fourth.

Obama launched his White House run with an Iowa win in 2008. This time, Obama is the only Democrat running, but the party is holding caucuses anyway and he will address caucus-goers by video on Tuesday night.

Santorum, a former U.S. senator from Pennsylvania, is the latest candidate to rise in Iowa polls in a race that has seen a handful of hopefuls roll through the top spot. His conservative views on issue like gay marriage and abortion make him popular with evangelical Christian voters, a strong force in Iowa Republican politics.

Surveys show Romney performs best of all the Republicans in head-to-head matchups with Obama in a general election campaign certain to focus on the economy and high unemployment.

Marcia Brom Smith, a Democrat and U.S. Navy veteran, attended a Romney rally in Des Moines on Tuesday to see if she would consider voting for him instead of Obama in the general election, but was not yet convinced.

I liked some of what he said. I don't think Obama should be blamed for the bad economy. The Congress is partly to blame in my view, she said. I did like what he (Romney) had to say about getting back to American values.

But some voters in Iowa, which has one of the nation's lowest unemployment rates, are torn between their urge to reclaim the White House from Obama and their search for a candidate who best meets their conservative principles.

(Additional reporting by Eric Johnson and John Whitesides in Iowa; Writing by Patricia Zengerle; Editing by Vicki Allen)