COLUMBUS, Ohio (Reuters) - Mitt Romney fought to open an unassailable lead over chief rival Rick Santorum in the race for the Republican U.S. presidential nomination on Tuesday, with Ohio the biggest prize among 10 states holding contests.
Romney, the winner of the past five state races, carried momentum into "Super Tuesday," the biggest day so far in the roller coaster Republican campaign. Some 419 of the 1,144 delegates needed to win the party's nomination are at stake.
Polls show Romney has effectively erased the more conservative Santorum's lead in Ohio, a traditional bellwether state that could play an important role in deciding the Republican nominee to challenge Democratic President Barack Obama on November 6.
Ohio is the most closely watched of the 10 Super Tuesday races, with contests also being held in Georgia, Massachusetts, Oklahoma, Tennessee, Vermont, Virginia, Idaho, Alaska and North Dakota.
A victory in Ohio and a good showing elsewhere would make Romney, a former Massachusetts governor, the favorite to win the nomination after a grinding battle in which he has been challenged by a series of conservative alternatives.
"If politics is a sport, then Super Tuesday is the decathlon, and we feel good about where things stand both nationally and in the states," said senior Romney aide Eric Fehrnstrom.
But a less-impressive showing could prompt renewed doubts about his ability to secure the nomination as Republicans continue their state-by-state battle to pick a nominee at their August convention.
Romney, a former private equity executive, has often struggled to connect with voters in a campaign that has focused on a successful business career that has made him a multimillionaire.
Santorum, a former senator from Pennsylvania, has staked out a platform that includes strong socially conservative positions. He has fought charges that his Senate career makes him a "Washington insider" and culture warrior who cannot appeal to Republican moderates.
"Santorum's values are more like mine - more conservative. I see Romney as more liberal and not sincere in his beliefs. He doesn't really know what he stands for," said Santorum supporter Katherine Frenz, 36, of Hilliard, Ohio.
Former House of Representatives Speaker Newt Gingrich, looking for a path to a comeback in the race, leads in his home state of Georgia. Ron Paul, a U.S. congressman from Texas known for his libertarian views, hopes to score his first win in Alaska.
Romney is favored in Virginia and Vermont and his home state of Massachusetts, and hoped to win Tennessee, where he was competing strongly with Santorum. The former senator is ahead in Oklahoma.
Obama, grabbing some of the Super Tuesday media spotlight, held a news conference that gave him a platform to highlight the successes of his presidency and push back against Republicans seeking to make him a one-term president.
Obama talked about help for struggling homeowners, rising gasoline prices, and growing tensions with Iran over its uranium enrichment program, which the United States and other Western nations fear has military aims.
Romney, Santorum and Gingrich all took time off the campaign trail on Tuesday to address the pro-Israel AIPAC lobbying group, each warning Iran over its nuclear program, which Israeli leaders regard as threat to the existence of the Jewish state.
"If Iran doesn't get rid of nuclear facilities, we will tear them down ourselves," Santorum told the influential group.
Obama sought to appear statesmanlike, drawing a contrast between his own careful approach as commander-in-chief and what he said was "bluster" and "big talk" from candidates on the campaign trail.
"If some of these folks think that it's time to launch a war, they should say so. And they should explain to the American people exactly why they would do that and what the consequences would be. Everything else is just talk," he said.
Romney was scheduled to fly home to Massachusetts to vote later in the day. Santorum was due to return to Ohio from Washington. Gingrich traveled to Alabama, which holds its primary contest on March 13, before heading back home to Georgia to push what opinion polls show is a flagging campaign.
"The only hope we have to beat Obama is to have better ideas communicated clearly and cutting through his billion-dollar campaign," Gingrich, who represented Georgia in Congress for 20 years, told a business group in the Atlanta area.
Santorum, who has a populist streak and a penchant for sweater vests, is sounding like a candidate who realizes he must win Ohio to retain credibility as Romney's main rival. He has been vastly outspent in Ohio by a Romney campaign machine that has pelted him with negative ads.
A win for Santorum in Ohio could flip the race upside down again.
While Santorum has hammered at Romney's conservative credentials, Romney has largely kept his focus on Obama's handling of the U.S. economy and foreign policy in the run-up to Super Tuesday, hoping to portray himself as the candidate best-placed to unseat the incumbent.
"I am voting for Romney because he is the lesser of all the evils - really I'm voting against Santorum and Gingrich," said Michelle McMahon, a Hilliard, Ohio, Romney supporter who is finishing up a masters degree in accounting and financial management.
Additional reporting by Eric Johnson in Hillier, Ohio and Colleen Jenkins in Atlanta; writing by Andrew Quinn; Editing by Ross Colvin and Eric Beech