Senate Republicans are discussing ways to block President Donald Trump from imposing tariffs on Mexican goods out of concerns over their impact on the U.S. economy — and the president said Tuesday the GOP would be “foolish” to do so.

“I don’t think they will do that,” Trump said during a news conference with British Prime Minister Theresa May in London. “I think if they do, it’s foolish.”

The president has vowed to apply tariffs of 5 percent on all Mexican goods next week and increase the rate to 25 percent in coming months if Mexico does not substantially halt illegal immigration across the U.S.-Mexican border, which is at a decade high this year.

Asked if Mexico, which has recently stepped up border apprehensions and deportations of Central American migrants, was doing enough to stave off tariffs, Trump said Tuesday that “it’s more likely that the tariffs go on” the

Republican senators, however, warned after a policy lunch with administration lawyers on Tuesday that the president doesn’t have the political backing to impose the tariffs, and that they could deal him an embarrassing rebuke if he decides to move forward.

“I think the administration ought to be concerned about another vote of disapproval on another national emergency act, this time about trying to implement tariffs,” Sen. Ron Johnson, R-Wis., told reporters after the lunch.