COLUMBIA, S.C. - After hearing that Mitt Romney said today he pays a tax rate "probably closer to the 15 percent rate than anything," Newt Gingrich called for the former Massachusetts governor to endorse his 15 percent flat tax plan.

"I'm thrilled at the idea that Mitt, I assume this afternoon will endorse my flat tax proposal and have every American pay it the way he paid," Gingrich said.

In an obvious jab at Romney, Gingrich also said he should rename his optional flat tax plan.

"I think we ought to rename our flat tax, we have a 15 percent flat tax, so this would be the 'Mitt Romney flat tax,' all Americans would pay the rate Mitt Romney paid, I think it's terrific," Gingrich said.

After questions and pressure from other candidates Monday at the Fox News/WSJ debate, Romney said he would likely release his taxes in April. Gingrich said today that April was not good enough for South Carolina voters.

"Well, what is he saying to the people of South Carolina, 'You're not important enough for me to release my income tax, nor are the people of Florida.' We'll get a list of every state he doesn't think is important enough," Gingrich said.

Romney indicated today that he would only be releasing his tax return from 2011, not from previous years. Gingrich said he wasn't suggesting Romney had something to hide, but he said he does not see how it is possible for a nominee to avoid releasing tax returns.

"Now, even if there's nothing there, so why isn't he releasing it, or there's something there so why is he hiding it, but this is kind of nonsensical," Gingrich said. "Oh, and he didn't say he would, he said he might."

Romney said today that Gingrich's plan to eliminate capital gains tax would provide people of very high income a possibility of no tax at all.

"You'd have individuals - the Warren Buffett argument - Warren Buffett, Bill Gates would probably pay no taxes at all. Today they probably pay 15 percent," Romney said.

ABC News' Emily Friedman contributed to this report.