Donald Trump had a public spat with Chuck Schumer and Nancy Pelosi the other day about funding for his border wall. Trump threatened to shut down the government if Congress doesn’t fund construction of the wall.

I confess I do not understand.

In 2015, Donald Trump kicked off his campaign for President with a speech characterizing Mexicans as criminals and rapists. He promised to build a border wall to deter immigrants and—crucially—to make Mexico pay for it. It was an insane promise: why would Mexico pay for the wall? Why would that nation even entertain the notion of quarantining itself behind a wall as if it were diseased? What possible inducement could Trump offer that would convince Mexico to debase itself in such a manner? What persuasive power could he have left after disparaging Mexicans in openly racist terms?

Whatever. He made the promise. And he kept making it, again and again. According to the Washington Post, from the time he announced his candidacy to today Trump publicly repeated his promise that Mexico would pay for the wall approximately 190 times. That promise was the centerpiece of his campaign, the raisin d’etre for his candidacy. Supporters chanted, “Build the wall” at campaign rallies. (They also chanted, “Lock her up,” another lunatic commitment that Trump could not keep and has not pursued, but that’s a subject for another day.)

I do not understand why Congress needs to provide money for a border wall when the money is supposed to come from Mexico. Not being a political consultant type, I also do not understand why Democrats do not talk about this every damned day. It seems so, so easy to me. Democrats should stop talking about the virtues of immigration, although such virtues are real. They should stop talking about why the wall isn’t necessary, or wouldn’t be effective even if it were built, although both of those things are true. They should even stop talking about the racism at the heart of Trump’s views on immigration, although that racism is real, and evil. Instead, they should have one message, and that message should be hammered home every time the subject of immigration is raised: “A promise made should be a promise kept. We welcome the President’s commitment to build a wall on our southern border with money from Mexico. As soon as funding from Mexico is secured, Congress will act to remove any legal obstacles to construction of the wall that may exist.”

Of course, there is no chance that Congress would ever have to remove any such obstacles, because there is no chance that Mexico will ever write the check. Thus, the message underscores the absurdity of Trump’s promise. It forces him to defend himself on ground that is indefensible. It highlights, again and again, the unkept promise. Democrats can claim they are in complete agreement with the President on the evils of illegal immigration, the necessity of a southern border wall, and the need for Mexico to fund it. And then they can sit back and watch as Trump falls on his face, either by trying to engage with Mexico over funding, or by trying to explain why he didn’t mean what he said.

Trump already has given ample evidence that he intends to make a fool of himself on this issue. His latest claim is that Mexico is going to pay for the wall by giving the U.S. more favorable terms in the re-negotiation of NAFTA. That means, of course, that any money gained by virtue of those more favorable terms would be diverted from the American economy to build the wall. Which means Americans would pay for it. His argument is as silly as if he had said he would get the richest one percent of Americans to fund the wall, but that they could do it by docking your paycheck. That kind of pretzel logic doesn’t pass the smell test.

When your adversary promises to do the impossible, you don’t try to talk him out of it. You encourage him to give it a go, because his certain failure is the quickest way to discredit him and his ideas. And it doesn’t require you to do anything but watch.