BOSTON (Reuters) - Mitt Romney's campaign told his Republican presidential rivals on Wednesday they could not catch him and nudged them to quit the race even though he failed to deliver a knockout blow in the biggest round of nominating contests.
Romney won six of the 10 "Super Tuesday" states, including a narrow victory in Ohio's marquee match-up, expanding his lead in delegates and solidifying his frontrunner status in the race to find the Republican challenger to President Barack Obama in the November 6 election.
But rival Rick Santorum won three states and Newt Gingrich captured one, keeping their hopes alive and raising the chance the divisive Republican fight could drag on for months. Both vowed they are in the race for the long haul.
Romney's wins on Tuesday gave him more than 400 delegates, according to many media counts, more than doubling Santorum's second-place total and moving him closer to the 1,144 needed to clinch the nomination at the party's August convention.
With few big voting days left in the Republican race, Romney's campaign advisers briefed reporters and issued a memo to argue that his rivals were fighting "basic principles of math" and had little chance to reshape the campaign given rules that award delegates proportionally in most of the remaining states.
"Super Tuesday dramatically reduced the likelihood that any of Governor Romney's opponents can obtain the Republican nomination," Romney adviser Rich Beeson said in a memo to reporters.
"As Governor Romney's opponents attempt to ignore the basic principles of math, the only person's odds of winning they are increasing are President Obama's," he said.
Santorum's campaign said Romney wanted the former Pennsylvania senator out of the race so he could move to the political center and abandon the conservatives who still distrust him for his past moderate positions on issues including abortion and healthcare.
"He wants us out so he can stop talking about conservatism," Santorum spokesman Hogan Gidley said. "There's a whole game to be played here. There are 28 states left. They are going to get a chance to voice their opinion in this race, too."
Santorum's allies urged Gingrich to quit the race so Santorum could consolidate conservative opposition to Romney. Gingrich won Georgia, the state he represented in Congress, but finished third behind Santorum and Romney in Tennessee and Oklahoma.
"Newt has become a hindrance to a conservative alternative," Stuart Roy, an adviser for the "Super PAC" political organization that backs Santorum, said in a memo.
The strong showing by Santorum on Tuesday underscored Romney's continued inability to win over large swathes of the party's core voters. But Romney, the former Massachusetts governor and private equity executive, said the party would unite once he secures the nomination.
"When we have a nominee we will come together because Barack Obama has organized the conservative community," Romney told CNBC, referring to the strong opposition to Obama's presidency among U.S. conservatives. "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.
He 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.
"We've got time, the resources and a plan to get all the delegates, and we think that will get done before the convention," Romney said. "There's not going to be a brokered convention where some new person comes in and becomes the nominee."
Romney's troubles with evangelical Christians and working-class voters are likely to persist in the next few contests. As the candidates spend millions of dollars attacking each other, polls show the lengthy nominating contest may be alienating voters.
The next nominating contest in the state-by-state battle for the Republican nomination is in conservative Kansas on Saturday, with the conservative southern states of Alabama and Mississippi voting on Tuesday.
Romney's strong organization and robust fundraising operation give him a strong leg up on his opponents.
In addition to Ohio, Romney won in liberal-leaning Massachusetts and Vermont on Tuesday, and also 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 said his victories in Tennessee, Oklahoma and North Dakota proved he was the best candidate to represent the party's conservative philosophy.
Exit polls showed that Ohio voters viewed Romney as more likely to defeat Obama, but thought Santorum was more sympathetic to average Americans' concerns.
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.
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 John Whitesides; Editing by Alistair Bell and Will Dunham