(AP) - Voting across 10 states marks the busiest day ofthe Republican race for president and is expected to determinewhether Mitt Romney solidifies his status as party front-runner orfaces further threats from his challengers.

Romney, a former Massachusetts governor, and former PennsylvaniaSen. Rick Santorum are locked in a tight battle for Ohio. FormerHouse Speaker Newt Gingrich hopes a big win in Georgia, which hehelped represent in Congress for two decades, will give new life tohis struggling campaign.

With 419 delegates at stake, Super Tuesday states offer asizable slice of the 1,144 required to clinch the GOP nomination.

Still, because of the apportionment of delegates based on votepercentage and the different regions the candidates areemphasizing, the race is expected to continue further into March.

Romney, who turned back Santorum in a close contest in Michiganlast week, hoped to continue his winning trend. He has won fourconsecutive contests, including Saturday's Washington caucuses.

After falling behind Santorum in Ohio last month, Romney hasclosed the gap in recent days, with polls showing the race a deadheat on the eve of the primary. It's a familiar trend for Romney,whose superior fundraising and turnout operation have helped himturn deficits in Florida and Michigan into triumphs.

The former venture capital executive kept his campaign's focuson the economy in a final sprint across Ohio, where he and Santorumare competing most fiercely.

"Other people in this race have debated about the economy,they've read about the economy, they've talked about it insubcommittee hearings," Romney said of his opponents. "But I'veactually been in it. I've worked in business and I understand whatit takes to get a business successful and to thrive."

Romney, the New Englander in the race, is expected to do well inthe Vermont and Massachusetts primaries. He is also poised to winthe Virginia primary.

Besides Ohio, Santorum is competing most aggressively inprimaries in Oklahoma and Tennessee, where the GOP's conservativehue matches the strict social conservative's evangelical appeal. Hewas leading narrowly in Tennessee, where polls showed Gingrich andRomney closing.

Despite signs that Gingrich planned to remain in the race,Santorum urged voters in Ohio to see it as increasingly atwo-candidate fight.

"I'm excited that we're here with the opportunity of winningstates on Super Tuesday ... and, hopefully, eventually, having theopportunity to go one on one at the end of this thing and see wherethis race really falls out," Santorum told supporters inMiamisville, Ohio. "And when we do, we'll win."

Gingrich has won only one state - the Jan. 21 South Carolinaprimary - and was projected to win only Georgia out of the 10states voting Tuesday. He began advertising in Tennessee on Monday,putting down just $35,000 for television time, a small purchase.

Yet, Gingrich planned to campaign Tuesday in Alabama, whichholds its primary March 13, even before the voting was finished inGeorgia. Ads for Gingrich were expected to begin airing in Alabamaand Mississippi, which holds its primary on the same day, and hewill visit both Southern states later in the week. He was thenheading to Kansas, which holds its caucuses Saturday.

Still, Gingrich tried to cast a likely win in Georgia as a signof momentum, comparing it to Romney's narrow win in his nativeMichigan over Santorum last week.

"It looks now like in Georgia we will carry the state by fouror five times the margin that Romney had Michigan," Gingrich toldsupporters Monday in Alcoa, Tenn. "We have a chance to win astunning victory in Tennessee."

Rep. Ron Paul of Texas was focusing on Tuesday's caucuses inAlaska, Idaho and North Dakota.