WASHINGTON – Mitt Romney won in six of 10 Super Tuesday states but managed only the narrowest victory over rival Rick Santorum in all-important Ohio, an outcome that leaves open the Republican contest for the nomination to challenge President Barack Obama.

Santorum, the former senator, captured three states and Newt Gingrich, the former speaker of the House of Representatives, notched a victory in Georgia, the state he represented in Congress for nearly two decades.

The mixed results in the 10 state contests held on Tuesday suggest that Romney, despite padding his lead in the tally of delegates who will pick the nominee this summer, is still struggling to win over the Republican party’s most conservative elements.

Regardless, said Eric Cantor, the No. 2 Republican in the House of Representatives, predicted Romney will win the nomination because his rivals “have not demonstrated an ability to do what needs to be done” to lock down the nod and Romney remains “the only candidate in this race who’s got a plan to turn this economy around.”

Speaking on CBS television Wednesday, Cantor’s remarks mirrored those of voters who told Ohio exit pollsters that the improving economy remained their chief concern.

Obama, who is vulnerable in his bid for a second term, has seen his approval ratings on the rise in recent weeks in tandem with increasing signs that the battered U.S. economy is finally on track for a sustained recovery from the worst downturn since the Great Depression in the 1930s. Prospects for Obama’s re-election also have improved as the Republicans’ campaign became mired the past two weeks in a bitter emphasis on social issues.

Sensing the opportunity to take some attention away from the Republicans, Obama held his first news conference of the year on Tuesday. He dismissed the Republicans’ almost-constant criticism of his foreign policy and accused them of “beating the drums of war” over Iran. “Those folks don’t have a lot of responsibilities. They’re not commander in chief,” he said.

But later in the day all attention again fell to the Republican race, especially in Ohio, where Romney’s victory on became certain in the early hours of Wednesday. And with Santorum and Gingrich energized by their own wins, the often-acrimonious state-by-state race will likely continue for weeks or months, perhaps weakening the eventual nominee and benefiting Obama, whose standing in opinion polls has improved.

Romney scored a home-state win in Massachusetts, where he served a governor, to go with primary victories in neighbouring Vermont and in Virginia, where neither Santorum nor Gingrich managed to get on the ballot. Romney later added the Idaho and Alaska caucuses to his column. Santorum won the primaries in Oklahoma and Tennessee and the North Dakota caucuses.

But Ohio, the heavily populated Midwestern industrial state, drew most attention given its history as a bellwether in presidential elections. It was a test of strength for Santorum, who was a senator from neighbouring Pennsylvania.

Romney trailed much of the night but rallied near midnight for a 38 per cent to 37 per cent victory over Santorum, an uncomfortably small margin for a candidate who had spent nearly four times as much money as his rival in the state. With virtually all precincts reporting, he led Santorum by about 12,000 votes out of more than 1.1 million cast.

Santorum, the latest candidate to pose a serious threat to Romney, saw Super Tuesday as an opportunity to prove that he still has a shot at the nomination. He needed to do well to curb Romney’s momentum from recent wins and huge financial and organizational advantage.

Santorum’s three wins and close-second in Ohio were impressive for a candidate few analysts took seriously just months ago. The success of Santorum, a Catholic, reflects his appeal to socially conservative Christians, an important part of the Republican base, particularly in the South.

“This was a big night tonight,” Santorum told cheering supporters in Ohio. “We have won in the West, the Midwest and the South, and we’re ready to win across this country.”

But Santorum is falling further behind Romney in the delegate count, and it was not clear how he can catch up. Moreover, Gingrich insists he will stay in the race after his win in Georgia. That will force a continued split among conservative voters who have refused to back Romney, the favourite of the Republican establishment whose past moderate record has drawn suspicion.

Libertarian-leaning Congressman Ron Paul drifted further back. He had pinned his hopes on winning caucuses in Idaho and Alaska but fell short in both.

At stake Tuesday were 419 delegates, more than a third of the 1,144 delegates needed to win the nomination at the party’s national convention in late August in Tampa, Fla.

Romney picked up at least 212 delegates on Tuesday, Santorum at least 84, Gingrich at least 72 and Paul at least 22, according to The Associated Press calculations. In the overall race for delegates, Romney leads with 415, Santorum has 176, Gingrich has 105 and Paul has 47.

The candidates now look to the next contests, Kansas and Wyoming caucuses on Saturday, and primaries March 13 in Alabama and Mississippi.