Mitt Romney won six of 10 Super Tuesday contests including crucial Ohio, advancing his claim on the Republican presidential nomination without ending questions about the breadth of his appeal within the party.
The former Massachusetts governor narrowly won in Ohio, the main event of the night, after earlier taking Idaho's caucuses and scoring primary wins in Virginia, Vermont and Massachusetts. He also picked up a win in Alaska.
Rick Santorum countered with wins in three states - Tennessee, Oklahoma and North Dakota- to keep his candidacy alive despite being heavily outspent by the front-runner.
Former House speaker Newt Gingrich won on home turf in Georgia, where he represented a district for two decades in Congress.
Santorum, the former Pennsylvania senator, scored substantial wins in GOP primaries in Tennessee and Oklahoma and won easily in North Dakota's caucuses, where Ron Paul finished a distant second and Romney finished third. Romney easily won primaries in his home of Massachusetts, neighboring Vermont and Virginia, where only he and Paul were on the ballot.
In Alaska, Ron Paul came in second.
"It looks like we're going to get a couple of gold medals and a whole passel of silver medals," Santorum told cheering supporters in Steubenville, Ohio. He acknowledged the results in that state were "still too close to call."
Romney, speaking to supporters in Boston before all the results were known, said he was looking forward to taking on President Obama as the GOP nominee even though Santorum's strong showing seemed to assure that the hard-fought GOP nomination race will continue.
"I'm going to get this nomination," Romney said.
"There are three states under our belt tonight, and counting. We're going to get more before this night is over," Romney said. "We're counting down the days to November, and that looks even better."
The candidates were vying for more than 400 nominating delegates at stake in the contests, which offered Romney the opportunity to solidify his role as front-runner in one of the most volatile GOP presidential nomination races ever.
Romney picked up at least 212 delegates during the night; Santorum got 84, Gingrich 72 and Paul at least 22.
Romney emerged from the evening with 415 delegates, more than all his rivals combined, a total that included previous state contests and endorsements from members of the Republican National Committee who automatically attend the party convention and can support any candidate they choose, according to the Associated Press's count. Santorum had 176 delegates, Gingrich 105 and Paul 47. It takes 1,144 delegates to win the nomination at the Republican National Convention in Tampa this August.
Ohio primaries dealt defeat to incumbent members of Congress in both parties. Four-term Republican Rep. Jean Schmit conceded the nominating primary in her Cincinnati-area district to an Iraq war veteran, Brad Wenstrup. Veteran Democratic Rep. Marcy Kaptur defeated longtime colleague Dennis Kucinich in an Ohio primary forced by a new congressional map that combined their districts. In the general election, Kaptur will face Republican primary winner Samuel Wurzelbacher, who gained the nickname "Joe the Plumber" for voicing working-class concerns during Obama's 2008 campaign in Ohio.
Gingrich and Santorum were battling to keep their presidential hopes alive, and Gingrich badly needed his Georgia victory to continue in the race. The fourth candidate, libertarian-leaning Paul, a congressman from Texas, had hoped to score a first win in Idaho, campaigning there on primary day, or Alaska.
"Thank you, Georgia," Gingrich said via Twitter. "It is gratifying to win my home state so decisively to launch our March Momentum."
Gingrich represented a Georgia district in Congress for two decades. He had the backing of the state's party establishment, including the endorsements of Gov. Nathan Deal, a former House member who served with Gingrich, and one-time presidential hopeful and pizza company executive Herman Cain.
All eyes were on Ohio, the battleground state that could be pivotal in the general election: No Republican president in modern times has been elected without carrying Ohio.
Santorum tried to capitalize in the big-population state with heavy industry as a next-door neighbor from western Pennsylvania, which shares rust-belt characteristics with much of Ohio.
Georgia actually had the most delegates to be awarded, 76, to Ohio's 66. Gingrich focused his attention there, though he now lives in Virginia.
In Virginia, only Romney and Paul submitted enough signatures to qualify for the presidential primary ballot.
Santorum's win in Tennessee reflected his emphasis on religion and social issues, an appeal to Bible Belt voters, while Romney ran second there. In Oklahoma, where Santorum also won, Romney and Gingrich were battling for second place.
All were vying for the chance to be the Republican Party's standard-bearer in the November general election against Obama, who faces no opposition for Democratic Party renomination.
With Ohio looming large in the Super Tuesday lineup, Jean Rauschenberg, 41, a stay-at-home mom and registered Republican from Bexley, Ohio, said she voted for Romney.
"Of the Republican candidates, I find Mitt is the closest to what I believe," she said. But in the general election, she said, she would probably support President Obama.
"I definitely have Democratic leanings," Rauschenberg said.
Textbook editor Heather Froelich, voting outside Columbus, gave her support to Romney, saying, "He understands the economy."
In many places, enthusiasm was in short supply among those casting ballots.
Gingrich got a reluctant vote from Tricia Tetrault in Edmond, Okla., where she explained her decision this way: "Ronald Reagan wasn't available anymore. What can I say?"
Santorum got the support of contractor Matt Howells in suburban Cleveland, but Howells didn't think his ballot would count for much.
"I really don't see a Republican winning the White House," he said.
With 437 delegates at stake in 10 states, Tuesday's voting represents a sizable slice of the 1,144 needed to nail down the GOP nomination. Wyoming caucuses, not considered part of Super Tuesday, began Tuesday and end Saturday.
Going into the Super Tuesday contests, Romney, who turned back Santorum in a close race in Michigan last week, had won four consecutive delegate contests, including Saturday's Washington caucuses.
After falling behind Santorum in Ohio in polls last month, Romney closed the gap in recent days, and polls showed the race a dead heat on the eve of the primary. It's a familiar trend for Romney, whose superior fundraising and voter-turnout operations have helped him turn deficits in Florida and Michigan into triumphs.