Romney, who turned 65 Monday, and his aides increasingly point to the delegate count as evidence that Romney will inevitably become the Republican nominee, said Santorum, who swept to a commanding victory in the Kansas caucuses Saturday.
But the race is more than "a mathematical formula," he told NBC's "Meet the Press."
Romney, a former Massachusetts governor, eked out primary wins in Ohio and Michigan, despite spending considerably more money than Santorum, the former senator from Pennsylvania said.
"In someone who's outspent 10-to-1, who has all the establishment behind him -- you know, all this, quote, wind to his back -- yet he can't close the deal, you know, winning Ohio, winning Michigan by the skin of his teeth," Santorum said.
By contrast, Santorum said, his own campaign has "been slowly crawling our way back -- clawing our way back into this race -- and, you know, we're in a great position right now as we go forward with states that are very favorable to us in favorable areas of the country."
The next Republican primaries are in Alabama and Mississippi Tuesday, followed by Illinois March 20.
Gingrich told "Fox News Sunday" that in Alabama and Mississippi, "I think we're probably polling ahead in both states right now."
He rejected the suggestion Santorum was the strongest conservative in the race.
"I think there is a space for a visionary conservative with big solutions, like national American energy policy and ... $2.50 a gallon gasoline, or a personal Social Security savings account for young Americans, or replacing the current 130-year-old civil services system with a brand new management model," he said.
Gingrich denied his campaign's suggestion he had to win Mississippi and Alabama to keep his candidacy alive, pledging to fight all the way to the Republican convention in August.
"Well, you know, the Mitt Romney camp has been trying to sell since last June that I should get out of the race and that Romney is inevitable," Gingrich said.
"But the fact is, Romney is probably the weakest Republican front-runner since Leonard Wood in 1920, and Wood lost on the 10th ballot," he said.
Wood was a four-star general who won the 1920 New Hampshire primary, but lost to Warren G. Harding at a contentious convention.
"Yes, he's the front-runner," Gingrich said of Romney. "He's not a very strong front-runner, and almost all conservatives are opposed to him, which is the base of the party.
"And I think we are as likely to see, after the last primary in June, we're likely to see a 60-day conversation about what's going to happen, as we are to see Romney nominated," he said.
Notable deaths of 2014 [PHOTOS]