WASHINGTON (AP) — Republican presidential candidate Newt Gingrich once led his rivals for the nomination in polls. Today, he's millions in debt and describing Mitt Romney as 'far and away the most likely' GOP nominee.

Running for president 'turned out to be much harder than I thought it would be,' he said Sunday.

'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,' Gingrich told 'Fox News Sunday' in a reflective interview.

After his Jan. 21 victory in the South Carolina primary, the former House Speaker said the Florida primary he lost in the following days turned into a 'real brawl.' He said Romney did a good job building a substantial machine, adding he has no regrets.

'Unfortunately, our guys tried to match Romney,' Gingrich said of the Florida match-up. 'It turned out, we didn't have anything like his capacity to raise money.'

Gingrich said he has a little less than $4.5 million in campaign debt, and he's operating on a shoestring budget.

Despite Gingrich's acknowledgment of what appears to be his inevitable defeat, the former House speaker Gingrich isn't ready to drop out. Gingrich wants to influence the party's platform, which is a statement of principles on the issues. He's interested in promoting increased domestic oil production and personal Social Security savings accounts.

But, if Romney secures the nomination, Gingrich said he'll campaign for him.

'I hit him as hard as I could. He hit me as 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 said.

Gingrich has had a campaign full of ups and downs. Just weeks after entering the race last year, his campaign imploded. Months later, in the weeks before the Iowa caucuses, he surged. He came in behind Romney and Rick Santorum in Iowa, but won South Carolina. He had several losses before winning his home state of Georgia. He had hoped to carry the momentum of that win to other contests in the South, so far unsuccessfully.