Super Tuesday: Romney edges out Santorum Ohio
US Republican presidential candidate Mitt Romney has crowned a string of Super Tuesday victories with a wafer-thin win in the swing state of Ohio.
As expected, he cruised to victory in his home state of Massachusetts, as well as Idaho, Vermont and Virginia.
Mr Romney also won in Alaska, which Ron Paul was pinning his hopes on for his only win of the nomination campaign.
Rick Santorum won a hat-trick of contests, while Newt Gingrich took his home state of Georgia.
Mr Romney now leads the field with 415 delegates committed to backing him at the national Republican convention in August. A candidate needs 1,144 delegates to win the party's nomination and go on to challenge Barack Obama in November's election.
But Super Tuesday did not deliver a sufficiently convincing victory to end the race and convince Mr Romney's rivals to pull out.
'Ready to win'
After Tuesday's 10-state voting marathon, Mr Romney defended his position as the front-runner.
"I'm going to get this nomination," he told supporters in Boston.
He easily won Massachusetts, where he was governor, as well as liberal-leaning Vermont and Idaho, where his fellow Mormons make up a chunk of the electorate.
Mr Romney also won resoundingly in Virginia, where Mr Santorum and Mr Gingrich failed to qualify for the ballot.
But Mr Santorum, a former US senator from Pennsylvania, said his victories in Tennessee, Oklahoma and North Dakota proved he was the bona fide conservative alternative to Mr Romney.
"This was a big night tonight," Mr Santorum told supporters in Steubenville, Ohio. "We have won in the West, the Midwest and the South, and we're ready to win across this country."
After a cliffhanger count, Mr Romney narrowly edged out Mr Santorum in Ohio, the night's most coveted prize.
Ohio was important because no Republican nominee has taken the White House without winning the Midwestern bellwether state in the general election.
Of the 66 delegates on offer, Mr Romney took home 35 compared to Mr Santorum's 21, the Associated Press reports.
Mr Santorum began the race in Ohio with a big lead in the opinion polls, but Mr Romney's well-funded political machine overcame him in part through a heavy campaign of attack adverts.
Mr Santorum has attracted the support of religious conservatives with his opposition to gay marriage and abortion.
But correspondents say his outspoken remarks on birth control and the role of religion may have turned off moderate-leaning voters.
Exit polls showed Ohio voters thought Mr Romney stood the best chance of beating Mr Obama, however, Mr Santorum appealed more to blue-collar voters.
Mr Gingrich, the former House speaker, did not achieve the sweep of Southern states he hoped for.
But he vowed to stay in the race after his Georgia win.
"There are lots of bunny rabbits to run through, I am the tortoise. I just take one step at a time," Mr Gingrich said.
With 96% of votes counted in Alaska, Mr Romney was winning with 33% of the vote, ahead of Mr Santorum with 29%. Texas Congressman Mr Paul - who had been hoping to make the state his only win of the campaign - was trailing with 22% while Mr Gingrich held 14%.
Of the 1,144 delegates needed to secure the Republican presidential nomination, 419 were up for grabs on Tuesday.
Overall, Mr Romney won at least 212 of Super Tuesday's delegates, taking his total to 415, while Mr Santorum added 84, taking his count to 176, AP reports.
The BBC's Paul Adams in Washington says the race is not over yet as the next crop of primaries and caucuses will not do Mr Romney any favours.
Kansas, Alabama, Mississippi and Hawaii hold their contests over the next 10 days.
Our correspondent says Mr Santorum and Mr Gingrich will be hoping to halt Mr Romney's momentum and keep their challenges alive.
The drawn-out nomination fight, which has been waged in large part through negative television adverts, may have taken its toll on the Republican Party.
A Washington Post/ABC News pollshowed only 35% of Americans looked upon Mr Romney favourably, compared to 32% for Mr Paul, 23% for Mr Gingrich, and 32% for Mr Santorum.