Republican rivals face first test of 2012 in Iowa DES MOINES, Iowa (Reuters) - Republican candidates crisscrossed Iowa in a last-minute bid for support on Tuesday ahead of the first contest of the 2012 U.S. presidential campaign, and at least three - Mitt Romney, Rick Santorum and Ron Paul - appeared to have a shot at victory. Iowa's caucuses are known more for weeding out candidates than picking the future president. Finishing in the top spot Tuesday night could provide a big boost in the state-by-state battle to select a Republican challenger to Democratic President Barack Obama in the November 6 election. Most of the candidates have topped opinion polls at one point in the volatile Republican race that until recently centered on televised debates rather than on-the-ground campaigning. Polls show Romney, the favorite of the party's business wing, in a tight race with Paul, a U.S. congressman from Texas with libertarian views, and Santorum, a former U.S. senator from Pennsylvania who hopes to consolidate Iowa's large bloc of Christian conservatives. Romney is a former Massachusetts governor. Many voters remain undecided. The unusual caucus process adds an element of unpredictability. Voters in Iowa gather in public meetings at hundreds of sites around the state such as schools, libraries and churches, listening to speeches touting the various candidates before casting their ballots. I'm kind of on the fence. A name that's been kind of sticking out, along with everyone, is Rick Santorum. We're not a hundred percent for sure yet," Des Moines resident Jason Harpineau said just hours before the start of the caucuses. Outside groups loosely allied with candidates, known as "Super PACs," have taken advantage of loosened campaign-finance rules to flood the Iowa airwaves with negative advertising. Former House of Representatives Speaker Newt Gingrich, who has seen his support erode under a barrage of such attack advertisements, lashed out at Romney for trying to distance himself from the ads. Asked if he was calling Romney a liar on the CBS program "The Early Show," Gingrich said: "Yes." 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. Romney said his campaign did not coordinate with the super PAC and told Gingrich to toughen up. "If you can't stand the heat of this little kitchen, wait for the hell's kitchen that's coming from Barack Obama," Romney said on Fox News. More than 100,000 voters - only a small percentage of the state's electorate - are expected to gather across the midwestern state at more than 800 public spots starting at 7 p.m. Results should begin coming in within a few hours. The weather was expected to be fairly cold, but dry, which should boost turnout. Democrats and independent voters will be able to participate if they register as Republicans at the last minute. Sparsely populated Iowa only yields 28 delegates of the 1,143 needed to lock up the Republican presidential nomination, and those delegates aren't actually awarded for months after Tuesday's caucuses. Still, the stakes are high. A strong performance in Iowa would provide visibility and a surge in donations to the winner or winners at the start of what is expected to be the most expensive election in history. Romney is aiming for a win that could ease persistent doubts among conservatives about his moderate past and propel him toward clinching the nomination early. He is heavily favored to win next week's New Hampshire primary. Surveys show Romney performs best among Republicans in head-to-head matchups with Obama in a campaign certain to focus on the economy and high unemployment. Santorum hopes to emerge as the latest conservative alternative to Romney. Largely consigned to the margins for most of the race, Santorum is now fending off attacks from his rivals who see him as a new threat. On Tuesday, he said Paul was behind a wave of automated phone calls that question his anti-abortion and pro-gun credentials. "Ron Paul is disgusting," Santorum told Fox News reporters. A win by Paul would help him extend his minimal-government stance and broaden the appeal of his campaign outside his zealous base, many of them independents, disaffected Democrats and younger voters. 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. The caucuses start a frenzied month 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. Iowa's nominating contest has traditionally cleared the field of losers and elevated surprise contenders. Former Arkansas Governor Mike Huckabee won the 2008 Republican caucuses, but fell short of the nomination. The eventual nominee, Arizona Senator John McCain, finished a distant fourth. Obama launched his White House run with an Iowa win four years ago. 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. Marcia Brom Smith, a Democrat and U.S. Navy veteran, attended a Romney rally in Des Moines 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. Additional reporting by Eric Johnson, John Whitesides and Lindsay Claiborn in Iowa; Writing by Andy Sullivan; Editing by Will Dunham