Super Tuesday: Romney and Santorum battle for Ohio
Republican presidential candidates Mitt Romney and Rick Santorum are locked in a tight battle for the state of Ohio, Super Tuesday's most coveted prize.
Mr Romney won the contests in Idaho, Massachusetts, Vermont and Virginia, while Mr Santorum prevailed in Oklahoma, North Dakota and Tennessee.
Meanwhile, Newt Gingrich declared victory in his home state of Georgia, and Alaska was still voting.
The eventual nominee will challenge Barack Obama in November's election.
The contest has gone down to the wire in Ohio, where just a few thousand votes separate Mr Romney and Mr Santorum.
Ohio has symbolic importance as no Republican nominee has ever become president without winning the crucial swing state in a general election.
'Tortoise and rabbits'
Front-runner Mr Romney's well-funded political machine has far outspent Mr Santorum in Ohio, chopping down his rival's double-digit opinion poll lead in the Midwestern bellwether state.
Mr Romney, a former Massachusetts governor and multi-millionaire businessman, has been riding a wave of momentum after poll wins in recent weeks.
At a rally in Boston, he attacked Mr Obama's stewardship of the economy, warned the president would be "unrestrained" if he won a second term, and said "I'm going to get this nomination".
But former Pennsylvania Senator Santorum - who bills himself as the true conservative in the race and a candidate who can win over blue-collar voters - has been hoping to halt his rival's winning streak.
"We're going to win a few. We're going to lose a few," Mr Santorum told cheering supporters in Ohio. "But as it looks right now, we're going to get a couple of gold medals and a whole passel of silver."
Mr Gingrich had not won a contest since South Carolina in January, and hopes his victory in his home state of Georgia will revive his campaign.
At a rally in Atlanta, the former House speaker lashed out at "elites" in the Republican party and the news media who he said had tried aggressively to drive him from the race.
"There are lots of bunny rabbits that run through. I'm the tortoise, I just take one step at a time," he said.
Texas Congressman Ron Paul, widely dismissed as a longshot candidate because of his libertarian-leaning views, is hoping to notch up his first win in Alaska's caucuses.
At a rally in North Dakota, he gave no indication of whether he would stay in the race.
Obama's well wishes
The BBC's North America editor, Mark Mardell, says there will be renewed questions about Mr Romney's appeal if he fails to win Ohio.
Many believe it is inevitable that Mr Romney will eventually be chosen to take on Mr Obama, our correspondent says, but few feel hugely inspired by the former Massachusetts governor.
This whole contest has been driven by the quest of the dominant conservative wing of the party to find somebody who is not Mr Romney, he adds, someone who is a true believer.
A devout Catholic who opposes abortion and gay marriage, Mr Santorum has sought to exploit lingering doubts among some conservatives about Mr Romney, a Mormon who governed in a liberal state.
Mr Romney won convincingly in Virginia, where Mr Santorum and Mr Gingrich failed to qualify for the ballot.
His victory in Massachusetts came as no surprise as he was governor there for four years. He was also the winner in Vermont, which is next door to his political homeland.
Mr Romney won as expected in Idaho, a state with a large population of his fellow Mormons.
Out of the 1,144 delegates needed to secure the Republican presidential nomination at the party convention in August, more than 400 are at stake in Tuesday's contests.