Newt Gingrich said today that he paid a 31 percent tax rate in 2010 and plans to release his tax returns for 2010 Thursday, followed by the 2011 documents.

"We can confirm that I paid a 31 percent rate, and although let me be clear, the 21st century Contract With America has an optional 15 percent for every American," Gingrich said at a press availability in South Carolina. "My goal is not to raise Mitt Romney's taxes. It's to let everybody pay Mitt Romney's rate. And so I'm not going to criticize Mitt Romney. I'm going to say, shouldn't we all have the option of a flat tax at the same rate he was paying."

Gingrich's taxes were in line with the total effective federal tax rate, which was 31.2 percent in 2010 for top 1 percent of taxpayers.

The former House speaker said his campaign went through his documents three times to make sure he paid a 31 percent rate, but said he would need to check whether that was only federal or both federal and state taxes.

"We're pulling together the documents," he added. "We hope to get it out sometime tomorrow. … And we'll release 2011 as soon as we put it together."

When asked what he earned, Gingrich said he did not know.

The former congressman divulged his tax returns after chief rival Mitt Romney revealed Tuesday that his effective tax rate is "closer to 15 percent."

Romney is under fire by his rivals - and has even been urged by his surrogates such as New Jersey Gov. Chris Christie - to release his tax returns. The former Massachusetts governor said in Monday night's debate that he will do so in April.

Gingrich, who has gained some momentum after the Fox News-Wall Street Journal debate, also said today that it would be helpful if Rick Santorum and Rick Perry dropped out of the race.

Gingrich has suggested that voting for either would only boost Romney because neither of them can win.

Santorum has dismissed those claims, pointing to his No. 2 finish in the Iowa caucuses, where he lost to Romney by eight votes, and his tie with Gingrich in the New Hampshire primary.

ABC News' Huma Khan contributed to this report.