Mitt Romney’s screed against President Trump is being taken as a sign that the newly elected senator from Utah may try to mount a challenge for the Republican presidential nomination in 2020. Good luck, we say. He’d be wise not to run on the character issue, though, because he’s just managed to forfeit the last shred of credibility he might have enjoyed on that head.

Mr. Romney issued his attack on Mr. Trump in an op-ed in the Washington Post. It hit the newsstands the day before the ex-governor of Massachusetts is due to be sworn in to the World’s Greatest Deliberative Body. “The president shapes the public character of the nation,” the Post’s headline said. “Trump’s character falls short.”

What a sucker punch. Two years ago, after all, Mr. Trump had just been elected President. Mr. Romney had been one of his most withering critics, delivering early in the campaign a speech attacking Mr. Trump’s character and, with words Ronald Reagan used in another context, warning that it was a time to choose. Almost no one urged Mr. Romney to get into the race, and no wonder, given how he’d led the GOP to defeat in 2012.

When, in November 2016, the country did choose, it delivered a landslide in the electoral college for Mr. Trump. How did Mr. Romney react? He fetched up at dinner with the President-Elect, where he wheedled for the job of secretary of state. “Romney Eats Crow at Three-Star Dinner With Donald Trump” was Vanity Fair’s headline. In the event, Mr. Trump chose a more gifted diplomat, Rex Tillerson of Mobil.

Soon Mr. Romney announced for Senate in Utah. He probably could have won without Mr. Trump’s endorsement, but maybe not if Mr. Trump had come out against him the way he had against Mr. Trump. The President, in any event, let bygones go and actually endorsed Mr. Romney for Senate. So the ex-governor of Massachusetts was headed to Washington from the Beehive State.

That’s the context in which Mr. Romney just clobbered Mr. Trump with the character issue. “Presidential leadership in qualities of character is indispensable,” he harrumphed. “And it is in this province where the incumbent’s shortfall has been most glaring.” In the history of the Republic it’s hard to remember any freshman launching his senate career with an attack on a President of his own party.

There’s only one reason anyone would do such a thing — it would be to maneuver for an attack in the next election. Mr. Romney has to be betting that a year or so hence, Mr. Trump will have been either driven from office or so badly wounded that he’s vulnerable to a challenge in the primaries. Mr. Romney can only be hoping that he will be in a premier position to take on a crippled president.

That in itself is not surprising. Mr. Romney has no other tactic left for national office beyond the Senate, particularly because he’s so full of praise for Mr. Trump’s policies and has already blown his chances for a cabinet job, at least while Mr. Trump is President. Ironically, in attacking Mr. Trump on character, Mr. Romney has given us a glimpse of his own character flaws.