Mitt Romney's campaign told his Republican presidential rivals on Wednesday they could not catch him and pushed 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 front-runner 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 improving the chances the divisive Republican fight could drag on for months.
The two men ignored Romney's calls to back down and vowed they are in the race for the long haul. For their part, Santorum's allies urged Gingrich to step aside and leave the former Pennsylvania senator as the conservative alternative to Romney.
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.
A senior Romney adviser told reporters it would take some kind of act of God for Santorum or Gingrich to catch Romney in delegates.
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.
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.
'28 STATES LEFT'
Santorum's campaign accused Romney of wanting him to drop out so he could move to the political centre and abandon conservatives who still distrust Romney 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 the former senator could consolidate conservative opposition to Romney. While Gingrich won Georgia, the state he represented in Congress, he finished third behind Santorum and Romney in Tennessee and Oklahoma.
Based on his electoral performance last night and his out-of-step record it is time for Newt Gingrich to exit the Republican nominating process, said Stuart Roy, an adviser for the Super PAC political organization that backs Santorum. Newt has become a hindrance to a conservative alternative.
With few big voting days left in the race, Romney's campaign advisers said his rivals had little chance to change the course of the campaign given rules that award delegates proportionally in most of the remaining states.
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.
'WE'VE GOT THE PEOPLE'
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.
But he hailed his 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.
Romney's troubles with evangelical Christians and working-class voters are likely to persist in the next few contests, however. As the candidates spend millions of dollars attacking each other, polls show the lengthy nominating contest may be alienating voters.
The next 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.
This race is going to change again, Santorum told a campaign rally in Lenexa, Kansas, in suburban Kansas City. He said he now considered the Republican battle to be a two-man race with Romney.
They've got the machine, they've got the insiders and the big money and we've got the people, he said.
Romney's strong organization and robust fundraising have given him a big advantage over his opponents. His campaign, which has been burning through money quickly, announced it raised $11.5 million in February.
Romney and his Super PAC allies outspent Santorum in Ohio by four-to-one, and are off to a quick start in Mississippi and Alabama. Romney has spent $165,000 on advertising in Alabama and his Super PAC has spent nearly $2 million in the two states combined, a Republican media buyer said.
Santorum's campaign has spent $100,000 in the two states combined, while his Super PAC is not investing yet, the buyer said. Gingrich's campaign has not bought advertising in the two states but his Super PAC has spent about $800,000 combined.
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 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 highly politicized 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 incomes 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. In 2008, Romney won six of 21 state contests on Super Tuesday but left the race later that week to clear the way for eventual nominee John McCain.
(Additional reporting by Sam Youngman in Massachusetts, Lily Kuo, Emily Stephenson, Susan Heavey and Alina Selyukh in Washington and Colleen Jenkins in Atlanta; Writing by Matt Spetalnick and John Whitesides; Editing by Alistair Bell and Xavier Briand)