GOP Sen. Charles Grassley: “This is a misuse of presidential tariff authority.” (Tom Williams/CQ Roll Call)

UPDATE, 1000 GMT:

Mexico’s Foreign Minister Marcelo Ebrard warned Monday of dire economic consequences if Donald Trump proceeds with tariffs.

Ebrard said at a news conference in Washington that the threat will not affect Mexico’s approach to immigration: “Mexico is ready to work on issues of common interest. [But] the imposition of tariffs will have a counterproductive effect and would not reduce the migratory flow.”

Ebrard said he and other officials will spend the next several days trying to persuade the White House not to implement the tariffs on June 10.

“What are we doing? Diplomacy,” he said.

The Mexicans have been working with Trump’s son-in-law Jared Kushner on trade issues.

Republican legislators are discussing a possible vote to block Donald Trump’s planned tariffs on Mexico, linked to his anti-immigration campaign.

“People familiar with the talks” said the vote could also have the effect of blocking Trump’s “national emergency”, declared in February, for his Wall with Mexico. Trump has been trying to use the declaration for a grab of billions of dollars, mainly from the military, for the 30-foot-high barrier.

Last week Trump announced his plan to impose a 5% tariff on Mexican products from June 10. The charge will raise 5% per month to a maximum of 25% until Trump decides he is satisfied with Mexican efforts to halt migrants heading towards the US.

Trump issued the order despite objections from his senior advisors, including son-in-law Jared Kushner and Treasury Secretary Steven Mnuchin. They fear the measure could jeopardize Congressional ratification of changes to the North American Free Trade Agreement, reached with Mexico and Canada last autumn.

TrumpWatch, Day 862: How Trump Overruled Advisors for Mexico Tariffs

Legislators are considering whether to nullify the national emergency, removing the basis for both the tariffs and the money for the Wall.

Trump Defies Concern

Republican representatives are concerned that the tariffs, coming on top of Trump’s escalating trade war with China, are effective tax increases on US businesses and consumers. On Monday, they joined Democrats in warnings about the effect of undermining NAFTA, now renamed the US-Mexico-Canada Agreement

“I think this calls into question our ability to pass the USMCA, much less get it passed by Canada and by Mexico,” GOP Sen. John Cornyn said. “And so we need to put our heads together and try to come up with a solution.”

Senate Majority Whip John Thune explained, “We have a lot of members who are very concerned, I think, about where this is headed.”

And Sen. Patrick Toomey has introduced a bill requiring Congressional approval before a president imposes tariffs for “national security”.

White House officials have not decided how to respond, amid uncertainty over tariff implementation. They have not explained exactly what steps are expected from the Mexican Government, although there are signs that authorities south of the border are detaining and deporting more migrants.

Meanwhile, Trump took a break from his photo-opportunity visit to the UK to press his campaign:

Cornyn hoped Trump might be persuaded to pull back:

We don’t even know what the President’s ultimately going to do. I know he’s, sometimes in his frustration, expressed his intention to do certain things but after calm reflection and consultation with the members of the Congress has decided maybe to pursue a different course, so that’s what I hope would happen here.

But fellow Republican Sen. Joni Ernst said Trump appeared to be unmoved when she appealed to him over the weekend: “He’s a tariff guy.”