Senate Republicans are pushing back on President Trump’s plan to impose tariffs on Mexico. But if Mexican officials think these Republicans are going to save them from Trump’s tariffs, it’s time for them to think again.

So far, congressional Republicans have managed to remain bystanders in Trump’s trade wars. If anything, they thought they would be voting on a new free-trade deal, the United States-Mexico-Canada Agreement, that would lift tariffs on trade among the three countries. But if Mexico does not come to an agreement with the Trump administration, that’s about to change.

Until now, Republicans have not been forced to vote on Trump’s tariffs because Trump imposed them using provisions of our trade laws, including Section 232 of the Trade Expansion Act of 1962 and Section 301 of the Trade Act of 1974. But to impose tariffs on Mexico over its failure to stop the flow of illegal migrants, Trump is using a different law, the International Emergency Economic Powers Act of 1977. This law gives the president authority to “regulate” the importation of “any property in which any foreign country or a national” has an interest. But he can do so only “to deal with an unusual and extraordinary threat with respect to which a national emergency has been declared.”

So, to impose tariffs on Mexico, Trump must declare a national emergency – and an emergency declaration means a vote in Congress on a resolution of disapproval.

Ah, you say, Republicans should be off the hook, because Trump already declared a national emergency on the southern border in February, which allows him to use military construction funds to build a border wall. In March, Congress passed a resolution of disapproval, Trump vetoed it and Congress failed to override it. No need to vote again, right?

Sadly, no. It turns out that the administration failed to reference the International Emergency Economic Powers Act as one of the laws underpinning Trump’s February executive order. So to impose tariffs under the 1977 law, Trump either has to (a) issue a brand-new executive order or (b) amend his earlier executive order to include the International Emergency Economic Powers Act. Either action triggers a new vote in Congress.

It gets worse. Many of the Republican senators who voted to support Trump on his emergency declaration in March say they would vote to disapprove of Trump’s tariffs today. Sen. Ted Cruz, R-Texas, who supported Trump in March, says he will oppose the Mexican tariffs because they represent a $30 billion tax increase on his constituents. And he’s not alone. Cruz told White House officials who briefed senators this week to tell the president that “you didn’t hear a single yes” from the Republican conference. Unlike his first executive order, this time there may even be enough Republican votes in the Senate to override a Trump veto.

Trump has warned Republicans that they would be “foolish” to try to stop him. And if he wants to make it difficult for them to vote against him, there is a simple way he can do so. If he issues a new order and Republicans vote to disapprove it, they will be voting only to stop the tariffs. But if Trump amends his existing order and Republicans vote to disapprove it, they will be voting to invalidate the entire emergency – terminating not just the tariffs but also his authority to build the border wall. That is the last thing Senate Republicans want to do.

Moreover, even if Republican senators did vote to override Trump’s veto of a disapproval resolution, it is not clear that House Republicans would join them in doing so – especially if such a vote were to strike down border-wall funding, as well. So, Senate Republicans could be left with the worst kind of vote – one that rebukes the Republican president but has zero effect on stopping his tariffs.

Mexico needs to understand that Senate Republicans won’t likely be riding to their rescue anytime soon. The Washington Post reports that Mexico is offering to dramatically increase its enforcement efforts and change asylum rules in an effort to stave off Trump’s tariffs. They had better offer a deal that can win Trump’s approval. The only way for Mexico to avoid a trade war that would be devastating to both of our countries is to reach an agreement with the president on securing the border – fast.

Washington Post columnist Marc A. Thiessen is a fellow at the American Enterprise Institute and former chief speechwriter for President George W. Bush.

Twitter: marcthiessen