Four years ago, Donald Trump became the first modern major-party presidential nominee to argue that his opponent should be incarcerated. The Republican struggled for months to say what it was, exactly, that Hillary Clinton did to warrant imprisonment, but “lock her up” nevertheless became one of the signature phrases of Trump’s quest for power in 2016.

Four years later, the president has struggled mightily to figure out what to say about Joe Biden. At various points over the course of the year, Trump has told voters to see the former vice president as “sleepy.” Or perhaps a “socialist.” Or “senile.” Or someone who had the audacity to move with his family to Delaware as a child.

Jon Favreau, one of Barack Obama’s former speechwriters, noted in the spring, “Good campaigns figure out one story to tell about their opponent. They might get there, but it’s May and [Trump campaign aides] haven’t figured it out yet.”

As the election season enters the home stretch, Trump has apparently decided, at long last, what he wants to say about his 2020 rival: the president wants voters to believe Biden, who’s never faced corruption allegations over the course of his lengthy career in public service, is a criminal. The Washington Post reported on Trump’s comments yesterday on a call with his campaign staff.

Trump also made a range of startling accusations and comments, including that Biden should be “in jail.” “He’s a criminal,” Trump said, without offering evidence what crime he had committed.

Soon after, during a brief tarmac Q&A, a reporter asked the president why he keeps accusing his opponent of being a criminal. Instead of answering in any meaningful way, Trump simply said, “He is a criminal. He’s a criminal…. Let me tell you something: Joe Biden is a criminal, and he’s been a criminal for a long time.”

The Republican proceeded to tell a group of supporters at an Arizona rally that the former vice president is lucky that Bill Barr is the current attorney general, “because I know people that would have had him locked up five weeks ago.” It came on the heels of a Georgia rally in which he similarly said, “Lock them up. You should lock them up. Lock up the Bidens.”

What’s more, two weeks ago, Trump publicly demanded that Barr prosecute his political opponents, including the former vice president before Election Day. “Unless Bill Barr indicts these people for crimes — the greatest political crime in the history of our country — then we’re going to get little satisfaction,” the president told Fox Business. “These people should be indicted … and it includes Biden.”

At no point has the Republican actually said which crime he thinks Biden committed. In fact, Trump seems to have stumbled onto this line of attack relatively recently, after other lines of attack proved unpersuasive and the Democrat’s lead in the polls grew.

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But in desperation, Trump has apparently come to the conclusion that falsely accusing an opponent of criminal misconduct worked for him once before, so he might as well try running the same play again. His 2016 antics made him sound like a tin-pot dictator, but he ended up in the White House anyway.

The trouble, of course, is that the president’s accusation doesn’t make any sense, and few are likely to take it seriously. With Clinton, voters were at least somewhat aware of a vague controversy surrounding an email server, which was the subject of FBI scrutiny. With Biden, there is no credible controversy.

Indeed, if the 2020 race comes down to which of the major-party candidates is more likely to need a strong criminal defense team in 2021, Trump will lose 50 states.

* Update: On Fox News this morning, the president kept the offensive going, saying, in reference to Attorney General Bill Barr, “He’s got to act, and he’s got to act fast. He’s got to appoint somebody.”