LONDON — President Trump on Tuesday said he plans to move forward with imposing tariffs on Mexican imports next week as part of his effort to stem the flow of migrants crossing the southern border, telling reporters at a news conference with British Prime Minister Theresa May that Republican senators would be “foolish” if they try to stop him.

“I think it’s more likely that the tariffs go on, and we’ll probably be talking during the time that the tariffs are on, and they’re going to be paid,” Mr. Trump said. When asked about Senate Republicans discussing ways to block the tariffs, Mr. Trump said, “I don’t think they will do that.”

He said, “I think if they do, it’s foolish.”

Mr. Trump’s latest threat — five percent tariffs on all goods imported from Mexico, rising to as high as 25 percent until the Mexican government stems the flow of migrants — has riled Republican senators who fear its impact on the economy and their home states.

But any vote to disapprove the tariffs would almost certainly face a presidential veto, meaning that both the House and Senate would have to muster two-thirds majorities to beat Mr. Trump. Opponents would use the same motion of disapproval that they tried to use to block the president from grabbing federal money for a border wall that was not appropriated for that purpose. That motion did pass Congress with significant Republican support, but not enough to overcome Mr. Trump’s veto.