Trump Is Said to Have Overruled Kushner and Other Aides in Threatening Mexico With Tariffs

And she defended the decision as necessary to getting Mexico to take action, telling Fox News that the administration had for months sought help from Mexico.

“The president has been asking them for months to do that,” she said. “Now he is putting some measures in place that hopefully will get them to engage more so that they will start to help us in this process.”

The president on Saturday again insisted the tariffs were necessary, repeating his assertion in a series of tweets that Mexico has facilitated drug and human trafficking across the border and suggesting the economic punishment was deserved. “They took many of our companies & jobs, the foolish Pols let it happen,” Mr. Trump said, “And now they will come back unless Mexico stops the travesty that is taking place in allowing millions of people to easily meander through their country and INVADE the U.S.”

Republican senators have proposed legislation to curb Mr. Trump’s authority to impose tariffs, but they have not targeted the emergency powers law that he is proposing to use in this case. Despite these efforts, there is little hope that such legislation could overcome a presidential veto in the Senate and the House, where Mr. Trump has strong support among Republicans. Instead, Senate staff aides were hopeful that Mexico could find a way to appease Mr. Trump or that the threat could dissipate, as the idea of closing the southwestern border did this year.

Since the announcement, Mr. Kushner, who worked with the Mexicans to secure concessions in the new trade pact, has been trying to cool tensions and has held calls with Mexico’s foreign minister, Marcelo Ebrard. On Wednesday, Mr. Ebrard is expected to meet in Washington with Secretary of State Mike Pompeo.

But the clock is ticking, and the administration has not offered clear guidelines for what would constitute a resolution that would forestall the tariffs. In a call with reporters Thursday night, Mr. Trump’s acting chief of staff, Mick Mulvaney, said that determining what would be a sufficient response from the Mexican government would be at the White House’s discretion.

“We did not set a specific percentage, did not set a specific number. It’s a very fluid situation,” he said, adding that the White House would monitor the situation “day-to-day and week-to-week.”

“We are going to judge success here by the number of people crossing the border, and that number needs to start coming down immediately in a significant and substantial number,” Mr. Mulvaney said.