Most members of Congress, including many Republicans from states adjoining Mexico, have seen President Donald Trump’s 2,000-mile wall along the southern border as a political device, not a serious proposal. Rightly so: The practical difficulties are enormous, and the benefits, supposing it could be done, minimal. The real question throughout has been how much money would be wasted pretending this project might work.

That question seems all the more pressing now we know that even the president doesn’t believe in his wall.

Last week, a leaked transcript of a phone call between Trump and Mexican President Enrique Peña Nieto revealed that Trump understands that the wall is really just a political prop. Imploring his Mexican counterpart to play along, Trump said that the wall is “the least important thing we are talking about, but politically this might be the most important.”

So how many billions are Republicans willing to spend on pointless construction, environmental damage and seizure of private property to help Trump save face?

The answer may be a lot.

The House Appropriations Committee in July proposed a Homeland Security spending bill that included $1.6 billion for a border wall. That’s a lot of money for the “least important thing” on the U.S.-Mexico agenda — while other, actually vital, U.S. domestic programs face cuts. It’s also $1.6 billion more than nothing, which is what Trump promised the wall would cost U.S. taxpayers, because he was going to make Mexico pay.

Republicans willing up to now to go along with this charade ought to stop and think.

It’s right to make illegal immigration harder (even though illegal border crossings from Mexico into the U.S. recently hit a 17-year low). But there are much cheaper and far more effective ways to do it.

More enforcement personnel and better technology make sense — both to deter smuggling and to facilitate commerce. Drones and ground sensors can be deployed to meet specific local conditions. In some cases, upgrades can be as simple as a paved road or an observation tower.

What can’t be justified is the wall. Trump has just acknowledged as much. Republicans need to take him at his word.