A coalition of conservative and free-market groups on Tuesday urged new U.S. Trade Representative Robert Lighthizer to reject any efforts to "withdraw from or materially weaken" the North American Free Trade Agreement.

The group said it was alarmed by President Trump's brief effort last month to start pulling the U.S. out of the agreement, arguing that would be a "catastrophic mistake."

"In recent weeks, President Trump has threatened to withdraw the United States from NAFTA if renegotiation efforts prove unsuccessful. This would be a catastrophic mistake. Since ratification, NAFTA has been an enormously beneficial, impactful arrangement for American businesses and consumers," said the coalition in a public letter to Lighthizer Tuesday.

"This said, we acknowledge that NAFTA may need to be modernized to reflect the realities of 21st century industry."

The signers included the American Legislative Exchange Council, Club for Growth, Competitive Enterprise Institute, FreedomWorks, National Taxpayers Union and R Street Institute.

While Republicans have traditionally backed free-trade deals, Trump has been a longtime critic of NAFTA and often slammed it on the campaign trail. He has continued to denounce it since taking office. In mid-April, President Trump told workers at a tool factory in Kenosha, Wis., "We are going to make some very big changes or we're going to get rid of NAFTA once and for all. It cannot continue like this, believe me."

On April 26, Trump abruptly decided to sign an executive order starting the pullout process. Commerce Secretary Wilbur Ross and Agriculture Secretary Sonny Perdue led an effort inside the administration to get Trump to back down, which he did the following day. Personal appeals from the Mexican president and Canadian prime minister also played a part. The president himself said later that pulling out of the trade agreement would create too much of a "shock to the system" but has pointedly not ruled out leading the U.S. out of the deal.

During a March Senate hearing on his nomination, Lighthizer said, "I agree with President Trump that we should have an America first trade policy and that we can do better in negotiating our trade agreements and stronger in enforcing our trade laws."