Mitt Romney reached out to conservatives suspicious of him and sought to assert himself as the inevitable Republican U.S. presidential nominee on Wednesday, despite failing to deliver a knockout blow to his rivals in the biggest round of nominating contests.

Romney won in six of the 10 states holding Super Tuesday primaries and caucuses, including a narrow victory in the most closely watched contest in Ohio. But rival Rick Santorum won in three states and Newt Gingrich captured one, raising the chances that the divisive Republican fight could drag on for months.

The results firmed up Romney's front-runner status for the party's nomination to challenge President Barack Obama, a Democrat, in the November 6 U.S. election. But the strong showing by Santorum underscored Romney's continued inability to win over large swathes of the Republican base, who question the moderate former Massachusetts governor's conservative credentials.

When we have a nominee we will come together because Barack Obama has organized the conservative community, Romney told CNBC. We're going to come together because we really believe that he needs to be replaced.

Romney's margin of victory was uncomfortably slim in Ohio, the night's biggest prize. Unlike some previous presidential campaigns, this year's Super Tuesday outcome failed to anoint a nominee.

Despite that, Romney hailed his Super Tuesday performance as a success and sought to dispel speculation among dissatisfied Republicans about new candidates jumping into the race or a brokered party nominating convention in Tampa, Florida, in August.

He tempered his words with caution, seeming to acknowledge the long battle still ahead for the nomination, and avoided bashing his Republican opponents, focusing instead on Obama.

We've got time, the resources and a plan to get all the delegates, and we think that will get done before the convention, he said. There's not going to be a brokered convention where some new person comes in and becomes the nominee.

With Tuesday's results, Romney methodically moved closer to the 1,144 delegates needed to win the party's nomination at the convention.

But his troubles with evangelical Christians 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 give him a strong leg up on his opponents.

SPLIT RESULTS

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

Santorum, a former U.S. senator from Pennsylvania, 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.

Gingrich, the former House of Representatives speaker, won his home state of Georgia and said he would stay in the race.

Romney narrowly beat Santorum in Ohio, which is considered a bellwether state, not only for the Republican nomination battle but also for the general election.

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

Romney, who built a fortune of at least $200 million (127.3 million pounds) as a private-equity executive, has struggled to connect with conservatives and blue-collar voters. A convincing win in Ohio could have put many of those doubts to rest.

Santorum 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.

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

Ron Paul, a U.S. congressman 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.

(Additional reporting by Sam Youngman in Massachusetts, Lily Kuo and Emily Stephenson and Susan Heavey in Washington and Colleen Jenkins in Atlanta; Writing by Matt Spetalnick and Andy Sullivan; Editing by Will Dunham)