(Bloomberg) -- President Donald Trump scoffed at the possibility that Senate Republicans would try to block his plan to impose a tariff on Mexico over concerns about the potential economic fallout.

“I don’t think they will do that,” he said Tuesday at a news conference with U.K. Prime Minister Theresa May in London. “I think if they do, it’s foolish. There’s nothing more important than borders.”

GOP lawmakers are considering whether to revive a resolution of disapproval against the national emergency declaration that underpins Trump’s justification for the tariffs, according to a person familiar with the matter. The person asked not to be identified in order to discuss sensitive deliberations.

Such an action could also stop the president from spending billions on a border wall without congressional approval, complicating any decision by congressional Republicans.

Congress earlier this year passed a similar resolution, but the House didn’t have enough votes to override a presidential veto. In dismissing the possibility of a rebuke by Congress on Tuesday, Trump cited his “tremendous” support within the GOP.

The political calculation for congressional Republicans could be different this time. Many lawmakers in Trump’s party are concerned about the economic impact of his Mexico tariffs, which will begin as a 5% duty on all of the country’s imports as of June 10 unless the country’s government takes unspecified steps to curb illegal migration to the U.S.

The tariff could rise to as high as 25% by October unless Mexico takes sufficient action, as judged by the president.

Earlier: Trump Issues First Veto on Bill to Block His Border Emergency

Trump left open the possibility that negotiations with Mexico could forestall imposition of the duties “but I think it’s more likely that the tariffs go on.”

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Senator John Kennedy, a Louisiana Republican, said he spoke with Trump Sunday about the Mexico tariffs and described him as “dug-in.”

“He’s as serious as four heart attacks and a stroke,” Kennedy told reporters at the Capitol. “A 5% tariff isn’t going to break the bank. A 25% tariff is a different story, but we are a long way from there.”

Discussions in Congress about a resolution of disapproval occurred as top GOP senators warned the administration that Congress could reclaim its tariff powers or hold up a new North American free-trade deal if the White House carries out its threat to impose the levies. It would take a critical mass of Republicans in the House and Senate publicly supporting a legislative rebellion to convince Trump to reverse course before the tariffs are set to begin.

Senate Finance Chairman Chuck Grassley, an Iowa Republican, suggested he won’t begin consideration of the U.S.-Mexico-Canada free trade agreement if the Mexico tariffs are put in place. Grassley used a similar threat to convince the White House to drop tariffs on steel and aluminum from Mexico and Canada earlier this year.

“It’s the same trouble that we had with the aluminum and steel tariffs. They have to come off before we can take it up here” he said of the USMCA.

Veto Override

House Speaker Nancy Pelosi has been working with her members and the Trump administration on the North American trade deal to address provisions on labor, the environment and enforcement that Democrats would like to strengthen. While Trump’s surprise tariff threat has thrown Mexico’s enthusiasm for the deal into doubt, Democrats say they still want to improve and pass the agreement.

“With respect to the USMCA we want to get to yes,” Hakeem Jeffries, the New York representative who chairs the Democratic Caucus, said Tuesday. Jeffries said Trump’s “tariff policy is erratic,” and the House will be watching the Republican senators to see how far they’re willing to go to oppose it.

Any measure that could be a check on Trump’s trade policy would need a two-thirds majority in both chambers to override a veto. That means roughly 20 Republican senators and 55 Republican representatives would have to stand up to the president -- and their constituents who support him.

The No. 2 Senate Republican, John Thune of South Dakota, said Congress could eventually act to curb emergency presidential trade powers if Trump carries out his threat to increase tariffs to 25% by October.

“I suspect Congress is going to want to be heard from, for sure,” Thune said. “We have a lot of our members who don’t like where this is headed, about what it means for the economy generally and don’t see it as a path to solve immigration issues.”

House Republican leader Kevin McCarthy expressed hope that the situation won’t get to the point of Congress being compelled to act.

"I believe that at the end of the day we will get to a solution that solves our problem,” he said at the Capitol.

Trade Authority

The reactions from members of Trump’s own party show just how far he has pushed Republicans who’ve traditionally supported free trade. The disagreement within the GOP is not just ideological -- there could be a political price to pay as well, especially in states where much of the economy depends on trade with neighboring countries.

Senator Mike Rounds, also from South Dakota, said that his state’s corn farmers are poised to feel the brunt of any Mexican retaliation.

“I think it is very fair to say that Congress should take a look at the authority they have sent to the executive branch, regardless of who the president is,” Rounds said. He added that South Dakotans want Trump to succeed but may need supplemental assistance if they are on the “tip of the spear” in fighting illegal immigration.

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Texas Senator John Cornyn said that any legislation curbing Trump’s powers would be difficult given his veto power.

“The challenges legislatively on this are obvious,” Cornyn said. “The president of the United States is going to have a say, so the best way forward is dialogue.”

Pushing Republicans

House Ways and Means Trade Subcommittee Chairman Earl Blumenauer said the House is also considering what action could get the support necessary to stop Trump’s Mexico tariffs. He said members of his committee will meet with House leadership Tuesday to discuss their options.

“There are a lot of members concerned that Trump has usurped this authority,” Blumenauer said.

Read More: Mexican Trade Delegation Meets with Trump Administration on Tariff Threat

Only a few Republicans have expressed any level of support for Trump’s move against Mexico, and frustration with the new tariffs was evident throughout a GOP caucus meeting on Monday.

Senate Agriculture Chairman Pat Roberts of Kansas called the tariffs “extremely counterproductive” and Colorado Senator Cory Gardner said they are “a bad idea, plain and simple.”

Republican senators have asked the administration for a briefing on the strategy. One specific concern is what Mexico must do to avoid the tariffs taking effect and scaling up.

“The market doesn’t like uncertainty and these kind of things create uncertainty, which is why you have seen the kind of volatility in the markets,” Thune said.

--With assistance from Josh Wingrove.

To contact the reporters on this story: Erik Wasson in Washington at ewasson@bloomberg.net;Daniel Flatley in Washington at dflatley1@bloomberg.net

To contact the editors responsible for this story: Joe Sobczyk at jsobczyk@bloomberg.net, Joshua Gallu, Alex Wayne

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