WASHINGTON — Republican presidential candidate Newt Gingrich, lagging far behind his rivals and his campaign mired in debt, acknowledged Sunday that Mitt Romney was his party's "most likely" nominee.

But the former House speaker insisted he was staying in the race despite being far outpaced by frontrunner Romney and fellow contender Rick Santorum, after polls once had him as the leading Republican to take on President Barack Obama in November's presidential elections.

"I think you have to be realistic, given the size of his organization, given the number of primaries he's won. He is far and away, the most likely Republican nominee," Gingrich told "Fox News Sunday."

He said that if Romney gets the 1,144 delegates to the Republican National Convention required to officially clinch the nomination, "I'll support him. I'll do everything I can this fall to help him defeat Obama."

Gingrich recognized that he owes "much more than we wanted to," with an estimated $4.5 million in debt, and that his campaign is "operating on a shoestring." He fired a third of his campaign staff just two weeks ago.

Running for president "turned out to be much harder than I thought it would be," the Georgia Republican said, expressing admiration for the substantial campaign machine Romney has built after an unsuccessful 2008 run, and adding that he has "no regrets."

"I hit him as hard as I could. He hit me hard as he could. It turned out he had more things to hit with than I did. And that's part of the business. He's done the fundraising side brilliantly," Gingrich added.

But he said he was not yet ready to drop out, even as his looming defeat grows increasingly inevitable. He has only managed to win Republican primaries in South Carolina and his home state of Georgia since voting got underway in January.

"I do think there's a desire for a more idea-oriented Republican Party, but that doesn't translate necessarily to being able to take on the Romney machine," he said.

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