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* Iowa kicks off Republican race with Paul, Romney at top
* Four conservative candidates battle for strong finish
* Unpredictable turnout could be key
By John Whitesides
DES MOINES, Iowa, Dec 31 (Reuters) - Republican voters in Iowa open a long and grueling 2012 presidential race on Tuesday, with polls showing Mitt Romney battling Ron Paul for a momentum-generating win in the party's kickoff nominating contest.
Four other contenders, led by surging Rick Santorum, are vying to consolidate conservative support and break into the top tier in the state-by-state fight to pick a Republican challenger to President Barack Obama, a Democrat, in November's election.
The stakes are high for each candidate, with Romney aiming for a win that could put him on a path toward clinching the nomination early and struggling rivals like Rick Perry and Michele Bachmann fighting to keep their White House hopes alive.
Newt Gingrich, leading the race just weeks ago until he withered under an onslaught of attack ads from Paul and an outside group aligned with Romney, needs a strong showing to stop his slide in the polls and show he is capable of mounting a comeback.
The candidates spent the last days of the Iowa campaign crisscrossing the state by bus in a burst of face-to-face campaigning, and flooding airwaves and mailboxes with millions of dollars in advertisements.
But polls showed at least four of every 10 Iowa Republicans entered the final stretch undecided or willing to change their minds about a frequently shifting presidential race that has seen the rise and fall of several contenders.
"You are starting to see some separation, with Paul and Romney moving into the lead and a real battle developing for third," said Tim Albrecht, an Iowa Republican strategist and aide to Governor Terry Branstad.
"But there is still a lot of uncertainty. Caucus campaigns in Iowa are notoriously tough to poll, and no one is sure who is going to turn out," he said.
Iowa's nominating contest has a spotty track record of picking winners, but it has traditionally winnowed the presidential field of laggards and elevated surprise contenders, setting the stage for later contests in New Hampshire, South Carolina, Florida and elsewhere.
The slide of Gingrich, the former House of Representatives speaker, has restored Romney as the race's frontrunner and presumed nominee.
Polls show Romney, the former governor of Massachusetts and former head of a private equity fund, 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.
A KNOCKOUT PUNCH?
A win in Iowa for Romney, an infrequent visitor to the state until this week's late push, combined with a Jan. 10 victory in his stronghold of New Hampshire would be a one-two knockout punch that could help him clinch the nomination early.
Paul, a congressman from Texas with libertarian views, has a dedicated core of followers and the best campaign organization in the state, but questions remain about whether he can expand his base of support and compete in states beyond Iowa.
The battle for the top spot among the four conservatives also could be critical. Republicans have struggled to settle on a conservative alternative to the more moderate Romney, who is distrusted by many on the right because of his past support for abortion rights and a state healthcare plan similar to Obama's federal overhaul.
Santorum, a former U.S. senator from Pennsylvania, has surged into third in recent polls, boosted by endorsements from prominent Iowa religious conservatives and the collapse of Gingrich.
But Santorum, who has run a shoestring campaign based on visiting all 99 Iowa counties and nurturing church-based support, would face a challenge in quickly raising enough money to compete in expensive later contests like Florida.
Perry, the Texas governor who has poured millions into television ads in Iowa, and Bachmann, a congresswoman from Minnesota whose poll numbers plunged after a summer turn at the top, need a good Iowa finish to replenish their campaign accounts and stay alive until the Jan. 21 contest in conservative South Carolina.
"Whoever comes in third will be competitive after Iowa, just given the conservative nature of Republican primary voters," said Steve Grubbs, an Iowa Republican strategist who was state chairman for former candidate Herman Cain.
"Iowa has never been about picking the winner, it's about winnowing the field," he said. "If Iowa sends a moderate, a libertarian and a conservative on to fight it out in the rest of the states, it has done its job."
Paul has remained near the top of Iowa polls despite a controversy over racially charged 1990s-era newsletters that appeared under his name. Iowa Republicans said the controversy has been slow to sink in with state voters preoccupied with the holiday season.
Much of Paul's strength comes from his appeal to independents and disaffected Democrats who might not have participated in past caucuses and from college students who will be on holiday break, making estimates of his turnout difficult.
The nature of Iowa's caucus system, which requires voters to head out on a cold winter night to join their neighbors at a community gathering spot for speeches and the balloting, also makes turnout unpredictable.
State activists agree, however, that Paul has Iowa's best organization, run by veteran members of the party's central committee.
"For all the talk about his unconventional ways, Paul set up the most conventional approach to Iowa, with a lot of retail politics and a really strong organization," said Steve Deace, an Iowa talk radio host.
"He's tilled the soil here, and when you till the soil you usually get a pretty good crop," Deace said.
The caucuses begin at 7 p.m. CST on Tuesday (0100 on Wednesday GMT). Relatively mild weather for Iowa is predicted for Tuesday night's caucuses, with temperatures slightly above freezing and no rain or snow expected. (For full U.S. election coverage, ) (Editing by Vicki Allen)