GOP Sen. Collins says she'll back resolution to block Trump's emergency declaration
Trump skeptical of anti-tax pledge
But a spokesman for Norquist says the billionaire businessman, currently leading in the GOP polls for president, has nothing to worry about.
Trump, in an interview with TIME, said he had "a problem" with the Americans for Tax Reform pledge because he "may want to switch taxes around."
Trump suggested he wanted to lower taxes on the middle class and said that "I have hedge fund guys that are making a lot of money that aren't paying anything."
Those comments are in line with Trump's broader populist message, including that the U.S.'s current immigration system has made it harder for average Americans to find jobs.
"I know a lot of bad people in this country that are making a hell of a lot of money and not paying taxes," Trump added in the interview, repeating that he wants "to put H&R Block out of business."
John Kartch, a spokesman for Americans for Tax Reform (ATR), said Trump "can cheerfully sign the pledge" despite those worries. Norquist is the founder and president of ATR.
Most GOP presidential contenders this year have signed the pledge to oppose and veto any tax increases if they're in the Oval Office.
Kartch suggested Thursday that Trump's ideas about shifting taxes around would be fine as long as he didn't increase the overall tax burden on Americans. Norquist has said in the past that reducing tax breaks in one part of the code must be offset elsewhere.
"Trump asked the right question: Can he do revenue neutral tax reform and be consistent with the pledge? And the answer is certainly yes," Kartch said.
Most GOP lawmakers in Washington have signed the pledge as well. But Norquist and former Florida Gov. Jeb Bush have battled in recent years over the pledge, especially after Bush endorsed raising taxes in a broad budget deal several years ago.
Bush now says he'll oppose any tax increases but has declined to sign the ATR pledge.