President Biden will headline the opening night of the Democratic National Convention on Monday with a valedictory speech that was supposed to have supercharged his final push for a second term but instead will serve as an opportunity to pass the torch to Vice President Kamala Harris.

Aides say Mr. Biden will use the speech to argue that Ms. Harris is the best person to carry on his legacy now that he is no longer in the race. Her candidacy as the party’s standard-bearer, he will argue, is the natural result of the choice he made four years ago to place her just a heartbeat away from the presidency.

That is not the case he expected to be making just over a month ago, before his stunning political demise following a debate performance in June that raised doubts about his age and vitality. Mr. Biden and his top aides, including Mike Donilon, his chief strategist, have spent the last several days reworking the speech for the new moment.

Mr. Biden will make only a brief appearance at the political celebration. His speech, which has a place of honor as the final one of the night, will begin just before 11 p.m. Eastern time. Around midnight, he and Jill Biden, the first lady, will fly to California for the start of a two-week vacation that will keep him out of the spotlight as Ms. Harris formally assumes her place as the new face of the Democratic Party.

The president will deliver his address to a convention hall filled with delegates who had been among his strongest supporters when he was a candidate. Organizers, who are decorating the hall with digital banners displaying common Bidenisms like “spread the faith,” expect an outpouring of affection for the 81-year-old president as he departs the stage.

But some are also bracing for potential disruptions among a few dozen uncommitted delegates who could use the speech as a moment to highlight their dissatisfaction with what they view as Mr. Biden’s lack of support for Palestinians in Gaza.

Aides say parts of the speech will be a review of the accomplishments his administration made in nearly four years that he believes provide the foundation to argue for another four years of Democratic rule. Before dropping out of the race, Mr. Biden regularly claimed that historians said he had accomplished more of consequence than any president since Franklin D. Roosevelt.

Mr. Biden will remind voters of the pandemic that had plunged the United States and the rest of the world into fear and economic chaos at the start of his term, and his efforts to return communities to normal. A large economic stimulus package and an embrace of vaccinations eventually pushed the virus into the background of the country’s consciousness.

He will focus on his domestic policy successes: low unemployment, millions of new jobs and legislation to protect the environment, repair crumbling infrastructure, lower the cost of some drugs and accelerate American investment in silicon chip manufacturing. He will also talk about his administration’s leadership abroad, including the efforts to counter Russian aggression against Ukraine.

But the president is also expected by aides to focus on the stakes of the 2024 election and what he sees as the existential danger if voters choose to return former President Donald J. Trump to the Oval Office.

Mr. Biden has long said he was motivated to run in 2020 by the threat that he believed Mr. Trump posed to basic democratic norms. In his speech, according to aides, he will argue that if voters elect Ms. Harris, it will mean that “democracy is preserved.”

The president’s address will be a bittersweet moment for him. Because of the pandemic in 2020, Mr. Biden’s convention was mostly held virtually, depriving him of the balloon-drop moment that he had sought several times during his life.

But this week’s convention — now built around Ms. Harris instead — will be a demonstration of the campaign machine that he assembled over the past year, and largely handed over to the vice president when he backed out of the race and endorsed her.