Spending billions of dollars to build hundreds of miles of additional walling — or “steel slats” or whatever you want to call it — on the US-Mexico border is a bad idea. That’s a critical, underrated feature of the current standoff that has led President Trump to partially shut down the government.

After all, if the president of the United States wants a $5 billion appropriation for a pet project that’s important to him personally and partially fulfills a campaign promise, then he ought to be able to get it. And the time-honored way to get it is to give congressional skeptics something else in exchange. That’s how the system has worked ever since Alexander Hamilton got James Madison to back federal assumption of state debts in exchange for locating the nation’s capital on the banks of the Potomac River.

But back at the beginning of the year, when it seemed as though a compromise involving wall money and a path to citizenship for DREAMers was in the works, it was immigration hardliners in Trump’s own administration who scuttled the deal. That’s certainly their prerogative, but it underscores the core truth of this standoff: Immigration hardliners themselves don’t think the wall is especially useful or important in the real world. If they really wanted a wall, they would go get a wall by offering something — it wouldn’t even necessarily have to be immigration-related — in exchange for it. But since they know the wall is a bad idea, they won’t trade it for anything.

Yet precisely because the wall idea is so bad, Democrats, rightly, aren’t going to give it away for free....