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 contests, carried momentum into Super Tuesday, the biggest day so far in the rollercoaster Republican race. 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.

Santorum, buoyed by conservatives' doubts about Romney's chances of beating Obama, vowed to keep fighting.

Ohio is the largest battleground and 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, the former Massachusetts governor, the favourite 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, the square-jawed former private equity executive, has often struggled to connect with voters in a campaign that has focused on a business career that has made him millions.

Santorum has staked out a platform that includes strong socially conservative positions, but has fought charges that his Senate career made 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, as she took her daughter to swimming lessons.

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 favoured in Virginia and Vermont and his home state of Massachusetts, and hoped to win Tennessee, where he was competing strongly with Santorum. The former U.S. senator from Pennsylvania is ahead in Oklahoma.

OBAMA WEIGHS IN

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 influential pro-Israel AIPAC lobbying group, each vowing to step up pressure on Iran and its nuclear program, which Israeli leaders regard as threat to the existence of the Jewish state.

Romney was scheduled to fly home to Massachusetts to vote later in the day. Santorum was due to return to Ohio from Washington. Gingrich took a swing 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.

You had better be prepared to wage a campaign of ideas this fall, because 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.

UPSIDE DOWN

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.

He has hammered at Romney's conservative credentials by highlighting the healthcare plan Romney developed as governor of Massachusetts and which Democrats say was a model for the national healthcare law signed by Obama. Republicans, Romney included, want to repeal Obama's healthcare overhaul.

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.

Republicans need a candidate who can go up against Obama, she said.

(Additional reporting by Eric Johnson in Hillier, Ohio and Colleen Jenkins in Atlanta; writing by Andrew Quinn; Editing by Ross Colvin and Will Dunham)