Mitt Romney's campaign told his Republican presidential rivals 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. But rival Rick Santorum won three states and Newt Gingrich captured one, keeping their hopes alive. 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 health care.

“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 next nominating contest in the state-by-state battle for the Republican nomination is in Kansas on Saturday, with Alabama and Mississippi voting on Tuesday.

In addition to Ohio, Romney won in Massachusetts and Vermont on Tuesday, and also in Idaho. 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.