Sen. Pat Toomey, R-Pa., is warning President Trump that he and other Republicans won't be bullied into abandoning NAFTA, and said Trump has no power to unilaterally withdraw from the agreement.

"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 wrote in an op-ed for the Wall Street Journal.

"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," he added in his op-ed, titled "Don't Try to Blackmail Us on NAFTA, Mr. President."

Trump has been pushing to renegotiate NAFTA to improve the deal for U.S. manufacturers and help bring jobs back to the U.S. But it's not clear Canada and Mexico will agree to a new deal, which has left Trump warning that he would move to exit the deal completely.

But Toomey's op-ed reminded Trump that Congress controls trade policy, and that there's no mechanism in the law for any president to withdraw.

"A president can no more repeal NAFTA than he can repeal ObamaCare or create a new NAFTA without Congress’s approval," he wrote.

Instead, Toomey said Trump should take on advice from lawmakers about how to update NAFTA. That includes looking for ways to get Canada to lower its barriers to dairy products, and get Mexico to open its energy market, he said.