Robert E. Lighthizer, U.S. trade representative nominee, listens during a Senate Finance Committee confirmation hearing in March. (Zach Gibson/Bloomberg)

Two Senate Republicans on Wednesday said they would vote against President Trump’s nominee for U.S. trade representative, questioning the White House’s adversarial negotiating tactics with Canada and Mexico.

Sens. John McCain (R-Ariz.) and Ben Sasse (R-Neb.) said in a joint statement that they would oppose Trump’s nominee, Robert E. Lighthizer, during his Senate confirmation vote on Thursday.

“Unfortunately, your confirmation process has failed to reassure us that you understand the North American Free Trade Agreement’s positive economic benefits to our respective States and the nation as a whole,” the lawmakers wrote Lighthizer in a letter.

Lighthizer, a senior trade official during the Reagan administration, is known for supporting tougher U.S. trade policy, particularly against China. The U.S. Chamber of Commerce and the steel industry have both endorsed his nomination, but concern among some lawmakers from both parties has grown about Trump’s trade agenda, particularly his threats to withdraw from NAFTA.

The Trump administration plans to soon send a letter to Congress that would formally begin the process of renegotiating NAFTA, but that process has been held up because Lighthizer still hasn’t been confirmed.

Republicans hold just a two-seat majority in the Senate, and Lighthizer’s confirmation could be imperiled if no Democrats support him during Thursday’s vote. But Democrats so far have not tried to mobilize an effort to block him. Lighthizer received unanimous support in April during a vote in the Senate Finance Committee, and Democrats have said it is important to have someone in that role soon to enforce U.S. trade rules.