Top Senate Republicans warned the Trump administration Monday 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 tariffs on Mexican imports because of immigration policy disagreements.

Senate Finance Chairman Chuck Grassley suggested he won’t begin consideration of the U.S.-Mexico-Canada free trade agreement if the Mexico tariffs go into 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.

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 per cent 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."

The reactions from members of Trump’s own party show just how far he has pushed his fellow Republicans, even if Congress has few legislative options that could reach a veto-proof majority. 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.

Trade Authority

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," said Rounds. He said 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 from Mexico.

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

“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.”

Iowa Senator Joni Ernst said that Congress may act but senators “need to sort through the legalities” of the authorities granted to each branch of government according to the Constitution and current law.

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 the Republican caucus on Monday.

Senate Agriculture Chairman Pat Roberts called them "extremely counterproductive” and Colorado Senator Cory Gardner said they are "a bad idea, plain and simple.”

For now, 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 White House said last week that the U.S. will levy a five per cent tariff on all imports from Mexico, unless that country takes action to halt migration from Central America. The tariffs will scale up every month until they reach 25 per cent in October, unless Mexico takes sufficient action, as judged by the Trump administration.

“I just hope they can reach some kind of agreement to delay this and continue to talk,” Thune said. “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.”