Republican presidential candidate Mitt Romney has crowned a string of primary victories with a win in the crucial state of Ohio.
Voters in 10 American states - Ohio, Georgia, Massachusetts, Tennessee, Vermont, Virginia, Oklahoma, Idaho, North Dakota and Alaska - chose their preferred candidate on what is known as Super Tuesday.
As expected, Mr Romney won in his liberal-leaning home state of Massachusetts, as well as Idaho, Vermont and Virginia.
After a cliffhanger count, he narrowly edged out Rick Santorum to take rust-belt Ohio.
The swing state is rated as the most coveted prize, as no Republican nominee has ever become president without winning the state in the general election, the BBC reports.
Mr Santorum scored convincing wins in conservative Tennessee, Oklahoma and North Dakota.
The last primary to fall to Mr Santorum was Alaska.
Newt Gingrich won his home state of Georgia.
Front-runner Mitt Romney, a former Massachusetts governor and private equity tycoon, has been riding a wave of momentum after poll wins in recent weeks.
But Mr Santorum, a former Pennsylvania senator, 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.
The primary election campaign lasts until about June, by which point one candidate has amassed enough delegates to win the nomination or has driven every other contender from the race, the BBC reports.
The eventual nominee challenges Barack Obama in the general election in November this year.