WASHINGTON - Republican presidential front-runner 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 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.

In the aftermath of the Tuesday vote, Obama campaign manager Jim Messina, in a conference call with reporters, said Romney was "limping across the finish line" in primary states. He added that the weakness of the Republican field was giving the president an "expanded map" of states to work with in the fall campaign.

Obama on Wednesday was in North Carolina, a state that figures prominently in the presidential election, calling for incentives to develop more fuel-efficient cars, emphasizing a top energy initiative amid rising gasoline prices and a re-election battle.

Gasoline prices are at their highest levels for this time of year and Obama has been travelling to promote energy proposals and is proposing a $1 billion incentive to challenge local communities to encourage greater fuel-efficient technologies, such as more charging stations for electric vehicles.

Vice-President Joe Biden next week will begin a series of speeches laying out themes for Obama's re-election campaign, beginning in Ohio. The Obama campaign also plans to release a 17-minute documentary on Obama's first term by director Davis Guggenheim, whose credits include the Academy Award-winning "An Inconvenient Truth," about Al Gore's global-warming campaign.

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.

Santorum, despite his victories, is falling further behind Romney in the delegate count, and chances he could catch up seemed diminishing. He must consolidate his hold on conservative voters who refuse to back Romney, the favourite of the Republican establishment whose past moderate record has drawn skepticism among the party's grassroots.

Santorum's chances were also hurt by Gingrich's win, which ensures he will stay in the race and continue to siphon away conservative voters.

In an appearance on CNBC, Romney insisted he's "getting the kind of support across the party that I need to become the nominee."

"We've got the time and the resources and a plan to get all the delegates, and we think that will get done before the convention," Romney said.

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.

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.

Wednesday morning, a super political action committee supporting Santorum said it was time for Gingrich to drop out. Stuart Roy, an adviser for the Red, White and Blue Fund, told The Associated Press that Gingrich was splitting conservative voters and making it difficult for them to settle on a conservative alternative to Romney.

Gingrich, in a morning appearance on Bill Bennett's "Morning in America" radio program, said he would have gotten out of the race if he'd lost Georgia. He also said there's no evidence Santorum could defeat Romney even in a one-on-one competition.

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, Florida.

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

On Wednesday the Romney organization said he raised $11.5 million in February, the second-best month ever for the campaign. He already has a huge advantage over fellow Republicans in the amount of money available and the scope of his campaign organization.

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