Sen. Pat Toomey wrote an op-ed in the Wall Street Journal on Thursday, promising to oppose President Donald Trump’s efforts to renegotiate or withdraw from the North American Free Trade Agreement (NAFTA).

President Trump continues to negotiate a revised NAFTA agreement with Canada and Mexico; the talks will likely finish before the July 1 Mexican elections.

Sen. Toomey argued that the Trump administration will either force the Senate to accept a “grim” choice between a “diminished” NAFTA or withdraw from NAFTA entirely.

Toomey wrote:

To pressure us into voting for an agreement that diminishes free trade, some in the administration suggest offering a grim choice: either approve a diminished Nafta, or the president will unilaterally withdraw the U.S. from the existing Nafta, leaving no Nafta at all.

Toomey then continued, suggesting that he will oppose Trump’s potential effort to withdraw from NAFTA altogether.

“If presented with this ultimatum, I will vote ‘no,’ urge my colleagues to do likewise, and oppose any effort by the administration to withdraw unilaterally,” Toomey said. “Pulling out of Nafta by executive fiat would be economically harmful and unconstitutional.”

Sen. Toomey wrote:

Unilateral executive withdrawal, however, would weaken our economy, diminish the institution of Congress, and undermine our constitutional responsibilities on trade. For all of those reasons, let’s hope the administration chooses an improved Nafta. The real disaster would be a Mexican standoff between Congress and the president.

Much to Toomey’s chagrin, a new Morning Consult poll suggested that 72 percent of Republicans support President Trump’s efforts to renegotiate NAFTA. The survey also found that a plurality of American voters, or 48 percent, back efforts to renegotiate NAFTA, 35 percent of Democrats, and 43 percent of Independents support renegotiating the NAFTA agreement.

President Trump has taken a tough approach towards NAFTA negotiations, suggesting that his tariffs aluminum and steel will only expire if the three North American countries sign a “new and fair NAFTA agreement.” Trump called NAFTA a “bad deal for the U.S.A.”

Toomey, who has often found himself at odds with President Trump on policy matters, has served as a fixture of the Republican establishment and has close ties to Wall Street. Toomey worked as a derivatives trader for Chemical Bank and Morgan Grenfell from 1984 to 1990. Toomey received $9,324,450 over his political career from the finance, real estate, and insurance industries.

In November, Sen. Toomey promoted amnesty for 3.5 million illegal aliens as a “package” deal, after admitting that such a compromise would lead to a “flood of new immigration.”

Toomey suggested last July that the reason that Republicans have had so much trouble repealing Obamacare is that he and other Republican lawmakers did not expect Trump to win the presidential election.

“Look, I didn’t expect Donald Trump to win. I think most of my colleagues didn’t, so we didn’t expect to be in this situation,” Sen. Toomey revealed.

Republicans have not unified on how they plan to address health care; many Pennsylvania citizens criticized Sen. Toomey and other GOP members for not adequately preparing to repeal Obamacare.

Toomey said at a town hall in July, “I think that’s a valid criticism.”