On Friday morning, President Trump took to his favorite soapbox, Twitter, and announced his latest plans for the southern border. In a series of tweets that started off with praise for Mexico’s cooperation with the U.S. to apprehend illegal immigrants, the president’s line swerved toward the absurd as he threatened new tariffs on cars, closing the border, and ended with a call for economic penalties for drugs smuggled across the border.

....However, if for any reason Mexico stops apprehending and bringing the illegals back to where they came from, the U.S. will be forced to Tariff at 25% all cars made in Mexico and shipped over the Border to us. If that doesn’t work, which it will, I will close the Border....... — Donald J. Trump (@realDonaldTrump) April 5, 2019

....This will supersede USMCA. Likewise I am looking at an economic penalty for the 500 Billion Dollars in illegal DRUGS that are shipped and smuggled through Mexico and across our Southern Border. Over 100,00 Americans die each year, sooo many families destroyed! — Donald J. Trump (@realDonaldTrump) April 5, 2019

The crowning jewel of the whole statement, however, was a consequential line tucked in among the threats: “This will supersede USMCA." Although more overt, Trump had already hinted at this possibility on Thursday when he floated new tariffs as an alternative to closing the border.

The USMCA, or United States-Mexico-Canada Agreement, of course, is the trade deal the Trump administration negotiated as a replacement to NAFTA. Trump, as one of the architects of the agreement, has been leaning heavily on lawmakers to pass the deal and has even threatened to withdraw from NAFTA to force their hand.

In a tweet, blasted off before heading to the southern border, however, Trump derailed arguments in favor of his own deal in Washington. Why should lawmakers support a deal that the president is already eager to undermine? Trump's tweet implies to allies and rivals alike that a deal is no protection from the unilateral, tariff-happy whims of the president. That won't help the U.S. reach a solid, lasting trade agreement with China or the European Union.