Mr. Cohen’s charges are serious, says Senate Democratic Whip Dick Durbin, but impeachment talk is “premature” because “more information has to come forward” and it’s “too early in the process to be using these words.”

Under the coy headline “Can Trump Survive?”—you already know his answer—Washington Post columnist E.J. Dionne counsels Democrats that “the argument for impeaching Trump suddenly became very strong, but this does not mean that turning 2018 into an impeachment election is prudent.”

And if you believe this misdirection, you probably also believe that Donald Trump didn’t canoodle with Stormy Daniels.

The political reality is that Democrats are all but certain to impeach Mr. Trump if they take the House in November. After what they’ve said and the process they’ve set in motion, Democrats won’t have much choice. They simply don’t want to admit this now before the election lest they rile up too many deplorables and independents who thought they elected a President for four years.

***

Let’s make the reasonable guess that Democrats retake the House with 228 seats, a narrow but solid 10-seat majority. They’ll have done so after two years of claiming that Mr. Trump is an illegitimate President who conspired with the Kremlin to steal the 2016 election, that he is profiting from the Presidency for personal gain, that he obstructed justice by firing James Comey, and that after Michael Cohen’s plea the President is now “an unindicted co-conspirator” in campaign-finance fraud.

If Democrats finally gain the power to do something about this menace to mankind, do they suddenly say “never mind”?

No doubt Democrats would start slowly by revving up the investigative machinery: subpoenas, hearings, all covered to a fare-thee-well by the media. Michael Cohen will be a major witness, as will the others named in the plea-deal documents. The Trump tax returns will get a star turn.

Once this starts, it will be hard to stop even if Democratic leaders want to. It will be even harder to stop if special counsel Robert Mueller writes a report to his superiors (that will inevitably leak) saying he couldn’t indict a sitting President but here is the evidence that he may have obstructed justice or have shady finances. The evidence may not even matter much since impeachment is a political process and Congress defines what are “high crimes and misdemeanors.”

Meanwhile, the battle for the 2020 Democratic nomination will be underway, with multiple candidates vying for the hearts and minds of liberal voters. They’ll compete to see who can be the loudest voice for impeachment. Even Terry McAuliffe, the former Virginia Governor who wants to run for President and who defended Bill Clinton against impeachment, has said impeaching Donald Trump is “something we ought to look at.”

There will be more-in-sorrow-than-anger calls for sober judgment, but political momentum has a mind of its own. The party’s liberal base will demand that Democrats be counted on an impeachment vote, and so will its media elites, who want vindication for believing that Mr. Trump could never have legitimately defeated their heroine.

The smarter political play might be to wait until 2020 and ride a potential wave of national fatigue with Mr. Trump, but don’t underestimate the degree to which liberals want this President to be politically humiliated and legally punished. Read their Twitter feeds and columns if you don’t believe us.

We don’t know how impeachment would play out politically in 2019 and 2020. An impeachment based on acts that have nothing to do with Russian collusion would offend much of the public, but as the New York Times joyfully put it this week, “that may not matter.” While a conviction in the Senate may seem improbable at this point, Democrats might not care because they’ll have made Republicans defend Mr. Trump’s behavior.

The main point about this election year is that no one should believe Democrats when they say that impeaching Donald Trump isn’t on their agenda. It’s their only agenda.