WASHINGTON – Defiant Republican senators warned Trump administration officials Tuesday that they were prepared to block the president’s effort to impose tariffs on Mexican imports, threatening to assemble a veto-proof majority to mount their most direct confrontation with the president since he took office.

During a closed-door lunch on Capitol Hill, at least six senators spoke in opposition to the tariffs President Donald Trump intends to levy next week in an attempt to force Mexico to limit Central American migration to the United States. No senator spoke in support, according to people present who spoke on the condition of anonymity to discuss the private meeting.

The lawmakers told officials from the White House and Justice Department that they probably had the Senate votes they needed to take action on the tariffs, even if that meant overriding a veto.

“There is not much support in my conference for tariffs – that’s for sure,” said Senate Majority Leader Mitch McConnell, R-Ky. He said senators hope that negotiations with Mexico will be “fruitful” and that the tariffs will not happen. Most GOP senators strongly oppose tariffs because they view them as taxes on Americans.

The contentious lunch meeting occurred hours after Trump, during a news conference in London, reiterated his intention to impose the tariffs next week and said it would be “foolish” for Republican senators to try to stop him.

“Mexico shouldn’t allow millions of people to try and enter our country, and they could stop it very quickly and I think they will,” Trump said at a news conference alongside British Prime Minister Theresa May. “And if they won’t, we’re going to put tariffs on. And every month those tariffs go from 5% to 10% to 15% to 20 and then to 25%.”

The escalating tension between Trump and Senate Republicans came on the eve of a critical meeting at the White House scheduled for Wednesday between U.S. and Mexican officials, led by Vice President Mike Pence and including an array of top officials from the Mexican government. The goal on the Mexican side is to head off the tariffs – the country sends 80% of its exports to the United States and counts it as its top trading partner.

But Mexican officials have been confused about what, precisely, the White House is demanding in exchange for the tariffs to be withdrawn, and White House officials will not say exactly what Trump wants. Some White House officials believe the meeting will mark the beginning of earnest negotiations that will pick up in intensity after the tariffs have been in place for a while. But many GOP senators view the imposition of the tariffs as unacceptable, and even as they hoped for a positive outcome from Wednesday’s talks they weighed their options for stopping the levies.

The exact process for a vote to block the tariffs remained unclear, but the basic scenario arises from the national emergency Trump declared at the southern border earlier this year to get more money for his border wall. Imposition of tariffs on all Mexican goods requires a legal justification, and administration officials say the existing emergency declaration could provide the basis for that, although it’s also possible Trump would declare a new emergency.

But the law that provides for presidential emergency declarations also allows Congress to vote to overturn them. When Trump declared the border emergency earlier this year, Congress voted to overturn it, but Trump vetoed the measure and Congress didn’t override the veto.

This time, opponents of Trump’s tariffs say they have enough support in the Senate to override a veto. If so, it would be the first successful veto override vote for Trump and a striking defeat for the president – even if the House ultimately sustains the president’s veto. A two-thirds vote is required in each chamber to override a veto, and Republicans in the House have shown scant interest in defying the president.

“I think the administration ought to be concerned about another vote of disapproval on another national emergency act, this time trying to implement tariffs. Tariffs are not real popular in the Republican conference,” said Sen. Ron Johnson, R-Wis., summarizing what he said he had told the administration officials at Tuesday’s lunch. “This is a different vote.”

At the lunch, senator after senator rose to confront the administration officials present – Steven Engel of the Justice Department’s Office of Legal Counsel and White House deputy counsel Patrick Philbin – according to several officials present in the room or briefed on the lunch who spoke on the condition of anonymity to describe it.

Sen. Rob Portman, R-Ohio, questioned the administration’s legal rationale for levying the tariffs, asking the officials to explain how it could use a law that has never been implemented to impose tariffs – the International Emergency Economic Powers Act – to, in fact, impose tariffs.

The officials assured the senators that they would relay their concerns to the White House but gave them little reason for optimism that Trump would change course, senators said.

Afterward, some Republicans emerged from the lunch convinced that opposition to Trump’s proposed levies on Mexico runs so deep that GOP senators could produce a veto-proof margin on a disapproval resolution.

“I sure do,” Sen. Kevin Cramer, R-N.D., said when asked whether he thought there would be at least 20 Republican votes to reject Trump’s tariffs on Mexico – which would constitute a veto-proof margin in combination with Democratic opposition. “There’s just a weariness of tariffs as the only tool in the toolkit that gets used.”

But senators were uncertain after the lunch about how Trump would proceed if he does impose the tariffs, and his actions would dictate their response.

“You could do this in a number of ways, and I think the White House has to decide,” said Sen. Roy Blunt, R-Mo., a member of Senate GOP leadership. “I think you could do this under the original emergency action if you wanted to, or you could do it as a separate action and we could wind up voting on a bunch of these every six months.”

Earlier, at the news conference in London, Trump said talks with Mexico will continue even as he goes forward with the tariffs, which he promised “will take effect next week.”

“It’s more likely the tariffs go on and we’ll probably be talking during the time the tariffs are on,” Trump said.

As for the GOP talk of voting on a disapproval resolution to block the levies, Trump said: “Oh, I don’t think they will do that. I think if they did it’s foolish.”

He added: “There’s nothing more important than borders. I’ve had tremendous Republican support.”

Trump shocked U.S. lawmakers and Mexican leaders last week by announcing that he would impose a 5% tariff on all goods imported from Mexico on June 10, and then increase the levies each month if Mexico doesn’t crack down on migrants.

GOP lawmakers warned White House officials that the tariffs could imperil the chances of passing an overhaul of the 1994 North American Free Trade Agreement, but Trump has remained undeterred.

Trump’s tone as he addressed reporters in London contrasted with that of Mexican Foreign Minister Marcelo Ebrard, who said at a news conference in Washington on Tuesday that he thought his country had an 80% chance of reaching a deal.

Mexico has undertaken a vigorous offensive to avert the U.S. tariffs. Its economic minister, its agriculture minister and others are meeting with U.S. counterparts, and delegations of Mexican lawmakers and business leaders traveled to Washington to warn against the tariffs.

Traditional pro-business Republican groups also have announced strong opposition to the tariffs. Americans for Prosperity, a group backed by the Koch network, sent a letter to congressional leaders on Tuesday calling the proposed tariffs “the largest tax hike in modern history” and saying that “it’s time for Congress to do its job.”

“Given the potential damage to our economy and our national interests, we believe it is time for Congress to reclaim its constitutionally mandated authority to impose tariffs, and to prevent further unilateral tariff increases by the president,” the letter said.

Mexican officials have indicated that they are willing to take new steps to restrain the movement of migrants to the U.S. border, although they have not described them publicly.

A Mexican official involved in the negotiations said the Trump administration has not submitted a list of specific actions it wants Mexico to take to avoid triggering the tariffs.

But Kevin McAleenan, the acting U.S. homeland security secretary, said the United States has conveyed to Mexican officials that it wants them to stiffen enforcement and dramatically reduce the number of Central Americans transiting the county en route to the U.S. border.

In an interview Tuesday with Hill.TV, McAleenan said the United States wants Mexico to harden its border with Guatemala and use the “natural choke points” created by the 125-mile-wide Isthmus of Tehuantepec in southern Mexico to screen vehicles and rail cars heading north. He said U.S. officials are also looking for Mexico to act upon intelligence it provides about the activities of criminal smuggling organizations. “We’ve given them all the information we have on the organizations that are moving migrants through Mexico that need to be dismantled,” McAleenan said.

U.S. authorities have detained more than 100,000 migrants along the Mexican border in each of the past two months. Mexico has nearly tripled monthly deportations since the start of the year but is struggling to cope with the rising flow of migrants.

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Sheridan reported from Mexico City. The Washington Post’s John Wagner, Nick Miroff, Paul Kane and Josh Dawsey contributed to this report.

Video: President Donald Trump last week threatened to impose tariffs on Mexican imports. The Washington Post’s Nick Miroff explains the significance.(REF:AkhtarM/The Washington Post)

Video: Senate Majority Leader Mitch McConnell, R-Ky., on Tuesday would not say whether Congress would vote to block President Donald Trump’s planned new tariffs on Mexico.(The Washington Post)

Video: At a Tuesday news conference with British Prime Minister Theresa May, President Donald Trump reiterated that tariffs on Mexico would go into effect next week.(The Washington Post)