Mitt Romney, the incoming senator from Utah and former Republican presidential nominee, revived his rivalry with President Trump on Tuesday with an op-ed essay in The Washington Post in which he said Mr. Trump “has not risen to the mantle of the office.”

Days before joining a Republican-controlled Senate, and as the 2020 presidential race begins to take form, Mr. Romney issued a pointed critique of the president’s character.

“With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable,” he wrote ahead of his swearing-in on Thursday. “And it is in this province where the incumbent’s shortfall has been most glaring.”

[After the essay, Mitt Romney faced counterattacks from the president’s allies. Read more here.]

The timing and tone of the piece set off widespread speculation online, with some suggesting that Mr. Romney aimed to position himself as “the new Jeff Flake,” the departing Republican senator from Arizona who publicly tangled with Mr. Trump.