WASHINGTON (AP) — President Donald Trump declared on Tuesday that fellow Republicans would be "foolish" to block the tariffs he's threatening on Mexican imports, but GOP senators fearing a new trade war were considering action and grilling his administration lawyers behind closed doors.

Republicans are deeply concerned that Trump's proposed 5% tariffs on all imports from Mexico would spike U.S. consumers' costs , harm the economy and imperil a major pending US-Mexico-Canada trade deal. Mexico is concerned as well, and top officials are in Washington working to stave off the threatened trade taxes.

If Congress should vote against the tariffs it would be a stiff rebuke to Trump, much like its earlier effort to reject money to build a long, impregnable border wall. But many on Capitol Hill remain hopeful talks this week between U.S. and Mexican officials will ease Trump away from the tariffs he's said will start next Monday.

"We're going to see if we can do something," Trump said during a press conference in London on the second day of his state visit to Britain.

"But I think it's more likely that the tariffs go on," he said. He also doubted Republicans in Congress would muster the votes against him. "If they do, it's foolish."

Mexico seemed much more optimistic about a resolution.

"By what we have seen so far, we will be able to reach an agreement," Mexican Foreign Minister Marcelo Ebrard said during a news conference at Mexico's Embassy in Washington. "That is why I think the imposition of tariffs can be avoided."

Ebrard arrived in Washington over the weekend as Mexico launched a diplomatic counteroffensive and fresh negotiations. On Tuesday, Mexico's trade negotiator Jesus Seade was meeting with U.S. Trade Representative Robert Lighthizer, and Ebrard will meet Wednesday with Secretary of State Mike Pompeo.

Republican senators are hopeful those talks will prevent the tariffs. But if negotiations should fail, the lawmakers warn they may have no choice but to take action in Congress to stop Trump.

GOP Sen. Rob Portman said Tuesday on CNBC that if the tariffs do go into effect next Monday as planned, "I do think Congress is likely to have a vote."

Portman was among those on Capitol Hill who worry the tariffs will derail the long-promised United-States-Mexico-Canada trade deal— a rewrite of the North American Free Trade Agreement that Trump campaigned against. The senator warned that Trump was "endangering" passage of his favored USMCA.

Sen. Chuck Grassley, R-Iowa, the chairman of the Finance Committee, told reporters Tuesday the tariffs make passage of USMCA "more difficult."

Questions remain, though, and senators invited Pat Philbin from the White House counsel's office and Steve Engel from the Department of Justice to their weekly lunch Tuesday to hear more about the administration's legal rationale for the tariffs.

Trump has indicated he will rely on the International Emergency Economic Powers Act to slap the tariffs on Mexican goods, a national emergency executive action he can take without congressional approval.

But lawmakers say if the president invokes a national emergency, they can vote on a resolution to disapprove. That's what happened earlier this year when lawmakers, stunned by Trump's claim of executive power, tried to block him from taking funds for the border wall with Mexico. Congress voted to disapprove of Trump's actions, but the president vetoed the resolution.

Yet it's unclear if Trump will, in fact, use the national emergency declaration or if opponents of the tariffs might resort to other legislative tools to block him.

"There's some disagreement even among Republicans," Grassley said. "We'd better get a legal answer."

While many Republicans who voted against Trump earlier this year actually supported his ultimate goal of building the border wall — but were uneasy with his executive reach to do it — the president doesn't have anywhere near the same backing for the tariffs.

In this case, Trump is using the tariffs as leverage against Mexico in his long-running battle to reduce illegal immigration.

Democrats -- and some Republicans -- doubt the tariffs will ever take effect. Senate Democratic leader Chuck Schumer said Tuesday that "Trump has a habit of talking tough and then retreating."

Trump claimed "millions of people" are entering the U.S. through Mexico and criticized congressional Democrats for not passing new laws. He said, "Mexico should not allow millions of people to try and enter our country."

It is unclear what more Mexico can do — and what would be enough — to satisfy Trump because the United States has not presented concrete benchmarks to assess whether the U.S. ally is sufficiently stemming the migrant flow from Central America.

Mexico calls the potential tariffs hurtful to the economies of both countries and useless to slow the northbound flow of Central American migrants.

"We need to put our heads together and try to come up with a solution," Texas Republican Sen. John Cornyn said Monday.

The tariff threat comes just as the administration has been pushing for passage of the USMCA trade accord. Mexico and Canada already have started the process of ratifying through their own legislatures.

On Monday, U.S. Commerce Secretary Wilbur Ross met with Mexican Economy Minister Graciela Marquez, and U.S. Agriculture Secretary Sonny Perdue hosted his Mexican counterpart, Victor Villalobos.

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Associated Press writer Padmananda Rama contributed.

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Luis Alonso Lugo on Twitter: http://www.twitter.com/luisalonsolugo