REMEMBER HOW President Trump was going to construct a wall along the border between the United States and Mexico — paid for by Mexico? There’s been a change of plans. Now Mr. Trump wants the Democrats to put up the cash.

All right; that’s a slight exaggeration. What’s actually happening is that the government’s current spending authority runs out April 29. Without a new bill, the country could face a partial shutdown of federal agencies. No one, Mr. Trump included, wants that, and the two parties are negotiating a deal to avoid it. However, his budget director, Mick Mulvaney, threw a wrench into the talks Thursday by declaring that the final deal should include money for Mr. Trump’s wall. Yes, Mr. Mulvaney said, Democrats “don’t like the wall, but they lost the election. And the president should, I think, at least have the opportunity to fund one of his highest priorities in the first funding bill under his administration.” A down payment on the wall might be the White House’s price for agreeing to the Democrats’ own priority: a key health-care subsidy for low-income consumers.

Republicans do not have the necessary 60 votes for passage of any spending deal in the Senate, which gives the Democrats leverage despite their minority status in both houses. They would be crazy to capitulate, and not only because the politics of the issue favor them. (Sixty-two percent of the public opposes building a wall along the entire border, according to a Pew Research poll.) As a policy matter, the wall is a foolish and wasteful enterprise, one whose legitimate purposes — stopping unauthorized immigration and drug smuggling — could be achieved at far lower cost through other means. In the unlikely event that this pharaonic enterprise ever did get completed, it would stand as a monument to the xenophobia Mr. Trump tapped to get elected. The sooner he can be forced to abandon it, the better.

Democrats are in the right on the health-care issue as well. At issue are billions of dollars to help lower-income health insurance exchange customers afford out-of-pocket expenses, money that the Obama administration provided but that the Republicans insist was not properly appropriated; they have a lawsuit pending on that point. Ideally, the Republicans would be abandoning that fight and engaging the Democrats in a genuinely constructive update of Obamacare, not the “repeal-and-replace” exercise they have been pursuing, without success, due to their own internal divisions. Intra-GOP talks are ongoing, with Mr. Trump suggesting that a compromise might be ready for a vote in the House next week. “The plan gets better and better and better,” Mr. Trump said, which is the opposite of the truth: Leaked versions of the bill would, under certain circumstances, allow states to let insurance companies sell policies that people with preexisting conditions could not afford.

April 29 also marks the 100th day of Mr. Trump’s presidency; he may be trying to conjure some sort of legislative victory before then, or at least put up a convincing show of trying. What he’s mainly demonstrating, though, are the reasons his accomplishments so far have been so paltry: His vaunted negotiating skills have delivered little, and his priorities have been misguided.