Super Tuesday 2012: Results could alter GOP presidential race
Romney, the New Englander in the race, is expected to do well in the Vermont and Massachusetts primaries.
He is also poised to win the Virginia primary.
Besides Ohio, Santorum is competing most aggressively in primaries in Oklahoma and Tennessee, where the GOP's conservative hue matches the strict social conservative's evangelical appeal.
He was leading narrowly in Tennessee, where polls showed Gingrich and Romney closing.
Despite signs that Gingrich planned to remain in the race, Santorum urged voters in Ohio to see it as increasingly a two-candidate fight.
"I'm excited that we're here with the opportunity of winning states on Super Tuesday ... and, hopefully, eventually, having the opportunity to go one on one at the end of this thing and see where this race really falls out," Santorum told supporters in Miamisville, Ohio. "And when we do, we'll win."
Gingrich has won only one state - the Jan. 21 South Carolina primary - and was projected to win only Georgia out of the 10 states 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, which holds its primary March 13, even before the voting was finished in Georgia.
Ads for Gingrich were expected to begin airing in Alabama and Mississippi, which holds its primary on the same day, and he will visit both Southern states later in the week.
He was then heading to Kansas, which holds its caucuses Saturday.
Still, Gingrich tried to cast a likely win in Georgia as a sign of momentum, comparing it to Romney's narrow win in his native Michigan over Santorum last week.
"It looks now like in Georgia we will carry the state by four or five times the margin that Romney had Michigan," Gingrich told supporters Monday in Alcoa, Tenn. "We have a chance to win a stunning victory in Tennessee."
Rep. Ron Paul of Texas was focusing on Tuesday's caucuses in Alaska, Idaho and North Dakota.
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