The collusion fantasy has officially given way to the impeachment fantasy.

The Left’s passionate investment in the Mueller investigation had much to do with shock and disbelief at Trump’s victory in 2016 and the hope of early deliverance — the special counsel probe as delectable revenge and deus ex machina.

The expectation that Mueller would blow Trump out of the White House with proof of collusion with Russia has, not surprisingly, come up empty. No worries. If Volume I of the Mueller report, on Russia, didn’t pan out, there’s always Volume II, on alleged obstruction.

When you are desperate to, in the memorable words of Democratic freshman congresswoman Rashida Tlaib “impeach this mother—,” any rationale will do.

If House Democrats impeach Trump, though, they will be sorely disappointed. They will wake up the day afterward and after all the drama and wall-to-wall coverage, he’ll still be president of the United States, tweeting per usual.

Nancy Pelosi, who has bizarrely become the backstop of reason in Democratic politics, is reluctant to go down this path. The question is whether she will get swamped by her base, just as the Republican House leadership did in 1998.

Impeachment has a long history in Anglo-America. The British statesman Edmund Burke called it “the great guardian of the purity of the Constitution,” and Cass Sunstein notes in his book on impeachment that the US Constitution probably wouldn’t have gotten ratified without a provision for it.

Yet it is widely misunderstood. Pelosi said in 2017 that the president can only be impeached for breaking the law, a claim that Trump also tweeted on Monday. This may be an understanding convenient for both of them, but it is incorrect.

“High crimes and misdemeanors” don’t have to be technically legal in nature. Alexander Hamilton explained in Federalist 65 that grounds for impeachment are “the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself.”

So it doesn’t matter that Mueller pulled up short of accusing Trump of a crime. More notable is how Trump’s machinations came to nothing.

He unsuccessfully tried to crimp an investigation that found no underlying offense, while his White House officially cooperated with the probe. We only know in such detail the episodes Mueller catalogues because the White House coughed up so much material and so many witnesses, up to and including the White House counsel.

What Trump is guilty of — shambolic, unfocused and ultimately ineffectual scheming — is closer to the term that the founders considered for the impeachment clause, “maladministration,” before rejecting it as too loose and vague.

Impeachment would be a symbolic mark against Trump, but at what cost? Impeachment won’t magnify the president’s alleged offenses, but make them smaller, as the argument devolves into a microscopic examination of his words and actions (and non-actions).

It would be the most forlorn impeachment ever. Andrew Johnson came close to getting removed. Richard Nixon quit before he got removed. Even with Bill Clinton, there was a moment when it seemed possible some Senate Democrats might flip against him.

With Trump, there is no chance of being removed by the Republican-held Senate, which would probably hold a perfunctory, minimal trial, underlining the absurdity of the effort. Trump’s approval ratings wouldn’t rocket skyward like Bill Clinton’s. But Democrats would suffer the opportunity cost of distracting attention from substantive issues people actually care about, and put their relatively moderate members in an awkward spot.

Then, there’s timing. We’re about 18 months before an election where voters can pronounce on Trump’s presidency directly, without assistance from House Judiciary Chairman Jerry Nadler.

In the end, there’s only one way to take back what Democrats believe was stolen from them in 2016. That’s to win the 2020 election, which will require some deftness, not perpetual grievance and enraged wishfulness.

Twitter: @RichLowry