A Mexican flag in Tijuana is seen behind an immigrant-rights protester near the U.S.-Mexico border wall in San Diego on April 21. (Mike Blake/Reuters)

In the annals of pathetic climb-downs, this Sunday-morning tweet from President Trump deserves a special place of honor, or perhaps dishonor:

“Eventually, but at a later date so we can get started early, Mexico will be paying, in some form, for the badly needed border wall.”

To put those weasel words in context, let me ask a question of all you small-business owners out there. If a customer brings some merchandise to the cash register and promises that one of his neighbors will pay you for it “eventually . . . at a later date . . . in some form,” what are your odds of ever seeing that money? How likely is it that “in some form” means cash? Do you let him walk out with the goods, or do you remind him you weren’t born yesterday?

Trump apparently believes we are all hopelessly naive. With his presidency nearing the 100-day mark, he is desperate not to have to acknowledge that his outrageous, ridiculous, impossible campaign promises were, in fact, outrageous, ridiculous and impossible.

There was nothing ambiguous about his pledge to build a wall along the southern border, with Mexico footing the bill. This is what Trump said in August following his meeting with Mexican President Enrique Peña Nieto, who has categorically denied that his country will pay a dime toward the barrier: “Mexico will pay for the wall, believe me, 100 percent. They don’t know it yet, but they will pay for the wall.”

(Jenny Starrs/The Washington Post)

Now, however, Trump is threatening to shut down the government or destabilize the health-insurance market if Congress does not appropriate $1.4 billion to begin work on the wall. So he wants U.S. taxpayers to pay for the thing, amid ever-more-vague promises that Mexico will ante up “in some form” at some future date. Like, never.

The idea of a 2,000-mile, 30-foot-high, “big, beautiful” wall along the entire border was always more of a revenge fantasy than an actual proposal. Still, it’s worth examining the impracticalities and impossibilities, because political debate is meaningless if it has no connection to the real world. Trump’s shtick about the wall doesn’t even rise to the level of truthiness, let alone truth.

Begin with the border itself. Most of the land on the U.S. side is owned by state governments, private citizens or Native American tribes. Given substantial opposition among those who live along the border to the idea of a wall, Trump would have to order the federal government to seize much of the needed land through eminent domain. Stories of fifth-generation ranchers being dispossessed will hardly go over well with the segment of Trump’s base that favors small government.

The desert terrain is forbidding. Billions of dollars’ worth of roads would have to be built to get construction equipment and materials to the border, causing untold environmental damage. The ecology of the region would be altered, with inevitable impact on wild animal populations.

This terrible idea comes at a time when net illegal migration across the border into the United States is close to zero. It also comes at a time when electronic sensing devices and aerial drones can provide a far more cost-effective way of monitoring and controlling the border.

It was a dumb idea from the start. But Trump evidently feels compelled to pretend that he’s actually going to build the wall as advertised. White House budget director Mick Mulvaney has actually threatened Democrats, of all people, that if they don’t support funding for the wall in spending legislation that must be passed this week, Trump will block subsidies that make it more cost-effective for insurance companies to participate in the Affordable Care Act exchanges.

Someone should remind the president that Republicans control both chambers of Congress; he should be threatening Paul Ryan and Mitch McConnell, not Nancy Pelosi and Chuck Schumer. Someone should also inform Trump that he runs the executive branch now — and if he chooses to sabotage the ACA, he’s welcome to the blame.

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Trump also promised a Muslim immigration ban, which the courts haven’t let him deliver. He said NATO was obsolete but now says it’s not. He promised health insurance “for everybody” but supports a bid by House Republicans to snatch it away from 24 million people. He promised to “drain the swamp” but refuses to address his own and his Cabinet members’ many conflicts of interest.

He lied to us, repeatedly and shamelessly. His supporters may not care — yet — but history is unforgiving. Yelling “fake news” cannot mask a fake presidency.

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