(Craig Lassig/EPA)

It’s become a pattern: Again and again, President Trump suddenly tosses out a major new policy idea, often in fewer than 280 characters, and our whole system is obliged to spring into action around it and pretend it’s real.

Trump regularly announces the imminent arrival of a “spectacular” new GOP health plan, compelling Republicans to spend a day or two pretending such a thing is in the works. Then there was Trump’s threat to completely shut down the border with Mexico, which officials had to pretend was something other than the lunatic impulse of our own Mad King.

This has now happened again, but with a weird twist: This time, Trump declared a crazy policy to be operative after members of his own administration privately decided it was unworkable and discarded it, thus forcing them to revive it and pretend it’s real even though they themselves already decided it’s a total nonstarter.

We’re talking, of course, about Trump’s new fascination with the idea of dumping undocumented immigrants into Democratic strongholds. Trump has now declared that it is his administration’s policy to ship them into “sanctuary cities” and states:

Let’s consider how this evolved. Last week, The Post reported that people in the White House, including immigration adviser Stephen Miller, came up with this idea. Sanctuary cities and states are places where local governments refuse to aid in arrests and deportations of the undocumented.

After this first came to light, both the White House and the Department of Homeland Security confirmed that it was debated internally but said it never went anywhere. “The idea was briefly and informally raised and quickly rejected,” said a White House spokesperson, while DHS added, “This was a suggestion that was floated and rejected, which ended any further discussion.”

Indeed, officials at Immigration and Customs Enforcement (ICE) rejected the idea out of hand, as indefensible policy that was quite possibly illegal.

It’s unclear whether the president knew about the idea before it became public. But then, after this idea outraged the right people — that is, after it “triggered the libs” — Trump then publicly embraced the concept. This forced the White House to reverse course and declare that it is indeed an “option on the table.”

Now Trump has declared it is policy.

The basic idea here is that the administration must release untold numbers of asylum-seeking migrants, due to either laws that prevent the detention of children or to detention resource constraints, or both, so it will bus them to sanctuary cities and states and drop them off there.

It’s unclear whether the Department of Homeland Security intends to implement the shift. Note that Trump’s tweet says the idea is “subject” to DHS, which seems to give the agency wiggle room not to implement it.

But the mere fact that Trump has declared this to be policy is itself remarkable, and we should not discount the significance of it solely because he routinely does so many things that are out of touch with reality and/or contemptuous of basic governing processes.

Trump has blithely and openly admitted that this idea is about retribution against Democrats. As he tweeted recently:

That’s a straight-up declaration that the motive behind the policy is that Democrats are unwilling to give him the legal changes (to make it harder to get asylum, and to make it easier to hold families indefinitely) that he wants.

In other words, it’s deliberately intended to troll his political adversaries, driven by an impulse to harm the part of America he has no desire to represent. (Note that the very presumption that this is something that Democrats should fear and hate is itself a highly problematic premise.)

What’s more, should it ever come to pass, it’s bound to exacerbate the very problem that has Trump in a rage. Wouldn’t there be a risk that the promise of getting transported to a locale with a governing structure that isn’t eager to help the feds get rid of migrants might make them more eager to come?

Finally, the policy involves the expenditure of public resources on what Trump himself admits is retaliatory and designed to pressure public officials of the opposing party into giving him his way — which is nakedly corrupt on its face.

Indeed, one key reason senior ICE officials had balked at the plan was that this would not be a “justified expenditure.” That is, it would almost certainly be illegal. But Trump has declared that the government has the “absolute legal right” to do this. Again, this almost certainly isn’t happening, but if it does, it will be fun to watch administration attorneys scramble to justify it.

We’re more than two years into the Trump presidency, yet the news media still isn’t sure what to do at moments like this. Every working journalist knows that it’s all but certain that this idea is fantasy. But in the meantime, we all act as though it’s something real whose merits should be analyzed and discussed. Because after all, he’s the president.

Read more:

Karen Tumulty: The Trump White House sets a new standard for cynicism and callousness

Max Boot: End the charade. Appoint Stephen Miller to run DHS.

Jenny A. Durkan: Seattle isn’t afraid of immigrants, Mr. Trump

David Bier: We have a new reason not to trust ICE

The Post’s View: The Trump administration’s arrogant crusade against ‘sanctuary’ jurisdictions