So let’s review. We’ve got one policy that enjoys broad public backing, as well as bipartisan support on Capitol Hill. We’ve got another policy that has little public backing, which congressional Democrats oppose implacably and leaves even some Republicans ambivalent. Kevin McCarthy, the House majority leader, said Tuesday that he hopes to punt the wall-funding question to December. Under these circumstances, making a DACA/wall deal would be like Rafael Nadal saying to his 75th-ranked opponent, “Sure, I’ll let you win half the time.”

I’ve been surprised in recent days to see some prominent liberal commentators venture that maybe the Democrats should take such a deal. What’s $1.5 billion, goes the argument? That won’t build a wall. In exchange, the Dreamers are safe.

It’s tempting. But once Democrats agree to one payment of $1.5 billion, the door to many payments of $10 billion or $20 billion has been flung open. And — here’s the crucial point — they will have lost all ability to make a principled case against Mr. Trump’s wall.