Mitt Romney eked out a close victory in Ohio but failed to land a knockout blow against rival Rick Santorum on the Republican presidential nominating contest's biggest night, raising the chances of a drawn-out battle between the party's establishment and its grassroots conservatives.

Republican presidential candidate, former Massachusetts governor Mitt Romney, greets supporters at his Super Tuesday primary party in Boston, on Tuesday. [Gerald Herbert / Associated Press]

Romney won six of the 10 state contests on Tuesday but his margin of victory was uncomfortably slim in Ohio, the night's biggest prize. Unlike previous contests, this year's "Super Tuesday" results failed to anoint a nominee.

Romney methodically moved closer to the 1,144 delegates needed to win the party's nomination, but a strong showing by Santorum underscored the front-runner's continued inability to win over large swathes of the Republican base, who view his past as a moderate Massachusetts governor with suspicion.

Romney's troubles with evangelicals and working-class voters are likely to persist in upcoming contests in the conservative states of Kansas, Alabama and Mississippi. As the candidates spend millions of dollars attacking each other, polls show the lengthy nominating contest may be alienating voters.

Still, Romney's strong organization and robust fundraising operation make him the favorite to face Democratic President Barack Obama in November. Super Tuesday may not have delivered a decisive verdict, but it did move Romney closer to the Republican nomination.

"Tonight we are counting up the delegates for the convention and counting down the days until November," Romney told supporters in his home state of Massachusetts.

Romney won in Massachusetts, as well as liberal-leaning Vermont and Idaho, where his fellow Mormons make up a substantial slice of the electorate. He also won in Alaska and Virginia, where Santorum failed to qualify for the ballot.

Santorum said his victories in Tennessee, Oklahoma and North Dakota proved he was the best candidate to represent the party's conservative philosophy. "In every state we overcame the odds," Santorum told supporters, noting that he had been consistently outspent by his rivals throughout the campaign.

Newt Gingrich won his home state of Georgia and said he would stay in the race.

With most of the votes counted in Ohio, Romney led Santorum by 38 percent to 37 percent, and TV networks projected he would take the state. Romney ran strongest in and around the state's largest cities, while Santorum carried the rural areas.

Exit polls showed that Ohio voters viewed Romney as more likely to defeat Obama, but thought Santorum was more sympathetic to average US citizens' concerns.

Romney, who built a fortune of at least $200 million as a private-equity executive, has struggled to connect with conservatives and blue-collar voters. A convincing win in Ohio would have put many of those doubts to rest.

Republican presidential candidate, former Pennsylvania senator Rick Santorum, speaks at his election night party in Steubenville, Ohio, on Tuesday. [Eric Gay / Associated Press]

Santorum, a former US senator from Pennsylvania, has won support of religious conservatives thanks to his opposition to gay marriage and his views on other hot-button social issues. His controversial comments about birth control and the role of religion have alienated moderate-leaning voters, especially younger women who will be a key constituency in November.

He has also focused on the white working-class voters who have moved increasingly to the Republican column in recent decades as their economic fortunes have stagnated.

"This campaign is about the towns that have been left behind," he told supporters.

Santorum's understaffed campaign also missed opportunities to qualify for delegates in Virginia and portions of Ohio. Romney and his allies outspent Santorum by nearly 4 to 1 in Ohio.

Gingrich's strategy of focusing on southern states did not pay off in Tennessee and Oklahoma, where he came in third.

Ron Paul, a US representative from Texas known for his libertarian views, had hoped to score his first win in Alaska, but came in a distant second behind Romney.

In recent presidential campaigns, the Super Tuesday wave of primaries and caucuses has often settled the Republican race. But this year's campaign is likely to stretch until April or May - or possibly until the last contest on June 26 - under new rules designed to attract more voters and boost enthusiasm.

The Republican contenders are viewed less favorably among voters at large than the eventual nominees in earlier contests.

Reuters