The Republican leaders fear a trade war that would dampen the economic benefits of their tax cuts, which the GOP is depending on to stave off heavy losses in November’s midterm congressional elections. Republicans were clearly hoping the White House would roll back Trump’s announcement over the weekend, either by putting off the tariffs or by making clear that key U.S. trading partners would be exempted. But the president gave no ground, defending his decision in a series of tweets and even welcoming a trade war as “easy to win.” He insisted that the tariffs would go away only if Canada and Mexico agreed to renegotiate the North American Free Trade Agreement. With Trump digging in, Ryan and his lieutenants tried a more confrontational approach.

“We are extremely worried about the consequences of a trade war and are urging the White House to not advance with this plan,” AshLee Strong, the speaker’s spokeswoman, said in a statement. “The new tax-reform law has boosted the economy and we certainly don’t want to jeopardize those gains.” Strong sent reporters an article blaming a dip in the stock market—whose previous gains Trump championed—on investor jitters over the president’s directive. Brady spent the weekend in Mexico and on television urging the president to narrow the tariffs, and on Monday, his office said he was gathering Republican signatures for a letter of concern to the president.

Trump still didn’t flinch. “No, we are not backing down,” he told reporters who asked about Ryan’s criticism in the Oval Office on Monday.

Congress could stop Trump from imposing the tariffs tomorrow if it wanted to. The Constitution gives the legislative branch explicit authority “to regulate commerce with foreign nations.” And just last month, on a 400 to 2 vote, the House passed legislation that extends for three years a program that reduces various tariffs for businesses.

But over the last 50 years, Congress has delegated the bulk of its trade power to the president, and there isn’t much expectation that it’ll wrest it back anytime soon. “To claw those powers back would in effect take veto-proof majorities coming out of the House and Senate, and I just don’t see that as remotely likely in the current circumstances,” said Edward Alden, a senior fellow specializing in trade at the Council on Foreign Relations. “So I think the president does hold all the cards here.” Trump is imposing the tariffs under a provision of the Trade Expansion Act of 1962 that allows the president to do so for reasons of national security. That rationale has rarely been used, trade experts said, and it could lead other countries to cite their own national security to restrict imports of U.S.-made products.

A Republican congressional aide, speaking on the condition of anonymity to discuss a sensitive policy topic, said GOP leaders “won’t rule out potential action down the line.” But the vagueness of that threat itself underscores the reluctance of Ryan and Senate Majority Leader Mitch McConnell—who has said nothing publicly on tariffs—to take Trump on so directly. To have any chance at success, they’d have to muster an overwhelming majority of Republicans, because Democrats remain deeply divided on trade. Even then they might fall short. When former President Barack Obama sought authority to negotiate trade agreements that would not be subject to amendment by Congress, all but 28 Democrats voted no. The trade deal he ultimately struck, the Trans-Pacific Partnership, never even received a vote.