Rick Santorum admitted  Thursday that his war chest had dried up by the time he'd reached Pennsylvania, and that his campaign  was in debt for the first time, convincing him he could not  compete against Mitt Romney in the Keystone State.

"We didn't have a lot of money to begin with, but we were at a point where we simply had in the last couple races, really worked hard and spent money, and particularly in Wisconsin, we felt we had to win Wisconsin in order to do well in Pennsylvania, and it was a situation where we simply didn't have the resources to compete going forward," Santorum said on the Family Research Council's Tony Perkins radio show Thursday, his first radio interview since he suspended his campaign Tuesday. "We for the first time in the campaign had a debt.  The debt was … more substantial than I was comfortable with."

Santorum, who said he defied the odds and ran a campaign "on a shoestring," disclosed for the first time that in the week after his loss in the Wisconsin primary he'd   "basically raised almost no money.

"It was a very very small trickle of funds that were coming in.  We realized it's one thing to go out and compete in Pennsylvania.  Romney had already laid down $4 million of advertising, and we were looking at probably not being able to spend a penny on advertising.  You just reach a point where you want to compete, but you have to be able to compete, and we felt we couldn't."

Santorum's strongest fundraising came after his Iowa win, and after his trifecta victories in February, when he won the Colorado, Minnesota  and the Missouri primaries. In the three days following those wins, the campaign raised  $3 million.  In February alone, it  hauled in $9 million. Its Iowa effort  ran on a shoestring, but after Santorum's virtual tie there,  the campaign raised $4.5 million  in January.

But  trying to compete with the Romney campaign's high-dollar spending in advertising throughout the primary and after an undoubtedly expensive  trip to Puerto Rico that failed to yield delegates, the campaign's funds had run out.

Despite having plenty of  possibilities to praise Romney during the interview, Santorum didn't take the opportunity,  nor did he talk about  future support. It's widely believed that Santorum will endorse Romney, even though Romney's name was noticeably missing from Santorum's withdrawal speech Tuesday.

The same afternoon that Santorum dropped out of the race, Santorum adviser John Brabender told ABC News that the former Pennsylvania senator  and Romney were already in contact and an endorsement was expected.

"They had a conversation," Brabender said. "They will have a meeting down the road. I would imagine a national endorsement would come up at that time. Clearly, the goal is to beat Barack Obama, and anything Rick can do to facilitate that, Rick will do."

On Wednesday evening, Santorum's biggest donor, Foster Friess, said he expected Santorum to support Romney, and he would do the same.

"There's a lot at stake," Friess said. "I've had some conversations with his people, but not with him directly about how I can help."