In 2016, Obama Passed a Baton. Tonight, He’ll Aim to Resurrect a Movement.

“Fair to say, this is not your typical election,” Barack Obama told a packed political convention about to make history by nominating a woman for president.

That was July 2016, as Mr. Obama was leaving the presidency, extolling the talents of Hillary Clinton, and warning of the dangers of Donald J. Trump, who was widely assumed by Democrats in the room to be easily beaten.

Anyone reading that speech today would realize instantly that Mr. Obama could give much of it, word for word, on Tuesday night in Chicago. The phrases will change, a reflection of the fact that Vice President Kamala Harris’s life story, and her experience in government, is dramatically different from Mrs. Clinton’s. But his core message about Ms. Harris’s tenacity may well be drawn from what he said about Mrs. Clinton.

There will doubtless be echoes of eight years ago, when Mr. Obama described the dangers posed by Mr. Trump, whom he called a peddler of “a deeply pessimistic vision of a country where we turn against each other, and turn away from the rest of the world.”

But Mr. Obama’s mission on Tuesday evening will be far larger than what he sought to accomplish in 2016. Then, he was handing off a baton, with the strength of the presidency behind him. This time, it will be his job to resurrect, and then reassemble, the kind of movement that propelled him to the White House.

And after President Biden’s farewell speech to the party on Monday, it is Mr. Obama’s job to separate Ms. Harris from the Biden years, while making the case that she was central enough to the Biden administration to slip seamlessly into the job — essentially the argument he made about Mrs. Clinton’s role in his own administration. And then he must seek to transfer to Ms. Harris the sense of endless horizons that surrounded his own first run for the presidency.