Trump reaffirmed that his new policy "will take effect next week" during a press conference in London alongside British Prime Minister Theresa May.
Asked by a reporter whether Mexico, which sent a delegation to the U.S. this week for talks with U.S. officials on the tariff and immigration disputes, has done enough to avoid the new duties, Trump said, "No, we haven't started yet."
When pressed to give his thoughts on Republicans who have signaled that they might take action to block the tariffs, Trump said, "I don't think they will do that."
"I think if they do, it's foolish," Trump added.
Multiple outlets reported that Republicans in Congress have met to discuss whether to go against Trump on his latest protectionist threat, which would steadily increase tariffs on all Mexican imports to 25% by October unless Mexico makes strides in stopping illegal immigration to the U.S.
Trump had initially tweeted that the flow of migrants illegally crossing into the U.S. would have to "STOP," but administration officials later said that there was no specific level of reduction they were looking for.
CNN reported Tuesday, however, that there was no clear GOP plan to handle the president's tariff threat, which could also jeopardize the new trade deal between the U.S., Mexico and Canada that is meant to replace NAFTA.
Even as some experts hypothesized that the White House wouldn't actually follow through on Trump's new policy, which would partly marry the fraught issues of trade and immigration, Trump sounded more and more committed to the plan over the weekend.
"Mexico is sending a big delegation to talk about the Border. Problem is, they've been "talking" for 25 years," Trump tweeted Sunday. "We want action, not talk."
The presser with May came as part of Trump's second visit to Britain, which coincides with the 75th anniversary of the D-Day allied invasion of Normandy, France, during World War II.
The event also came just three days before May was set to resign as U.K. prime minister, after struggling throughout her tenure to secure a deal for Britain to leave the European Union in accordance with the 2016 Brexit vote.
Trump told reporters at the press conference that "we'll see if we can do something" during the U.S.-Mexico talks this week.
"But 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," Trump said.
"Look, millions of people are flowing through Mexico," the president said. "That's unacceptable."