President Biden used his valedictory address at the Democratic National Convention on Monday to deliver a lengthy defense of his own record aimed at cementing a 50-year legacy of public service, even as he passed the reins to Vice President Kamala Harris as the new face of the party he led until just weeks ago.

Speaking to a raucous arena filled with Democrats who chanted “We Love Joe!” and gave him a three-and-a-half-minute standing ovation, Mr. Biden grew emotional as he delivered what amounted to a political farewell to his most fervent supporters.

“I’ve given my heart and soul to our nation, and I’ve been blessed a million times in return,” the president said as he made the case that Ms. Harris is the best person to lead the country now that he is no longer in the race.

“She’s tough. She’s experienced. And she has enormous integrity,” Mr. Biden said. Selecting Ms. Harris as his vice president, he added, “was the best decision I made my whole career.”

That was not the case that Mr. Biden expected to be making just over a month ago, before his stunning political demise following a debate performance that raised doubts about his age and vitality. And in some ways, his praise of Ms. Harris during the 52-minute address seemed secondary to his desire to recount the greatest hits of his career in a speech he might have given as the party’s nominee.

One by one, Mr. Biden ticked through a litany of achievements from the stump speeches of his now-ended campaign: $35 insulin; breathing room for American families; strengthening NATO; beating “big Pharma”; burn pits that harmed veterans; investments in infrastructure; appointing a Black woman to serve on the Supreme Court; and the now-familiar call to remember “who the hell we are: We’re the United States of America, and there’s nothing we can’t do if we do it together.”

And he recited a favorite phrase from “American Anthem,” a song of modern politics sung by the mezzo-soprano Denyce Graves for presidents of both parties.

“America, I gave my best to you,” he said, repeating a line he used during his inaugural address nearly four years earlier.

Describing his one-term presidency as a successful rescue of a country reeling from a pandemic and suffering from the effect of political violence, Mr. Biden denounced former President Donald J. Trump, the 2024 Republican nominee, as not worthy of serving in the White House during his first term — “not then, not now, not ever.”

Showing flashes of anger, Mr. Biden accused Mr. Trump of “giving oxygen to the oldest forces that had long sought to tear apart America,” recalling the praise that the former president offered after neo-Nazis marched in Charlottesville, Va., in 2017. Mr. Biden claimed credit for preventing Mr. Trump’s re-election in 2020.

“Democracy has prevailed, democracy has delivered and democracy must be preserved,” the president declared, speaking with a booming voice that echoed throughout the Chicago arena where Democrats gathered to nominate Ms. Harris in his place.

Mr. Biden took the stage at the convention after an emotional introduction by Ashley Biden, his daughter, who described him as “the OG girl dad.” The president wiped tears from his eyes as he approached the lectern.

In his remarks on Monday, he vowed to work for Ms. Harris’s election and urged Americans to do the same, describing the Democratic ticket of the vice president and Gov. Tim Walz of Minnesota as fully committed to the ideals that he embraced as a senator, vice president and president.

“Kamala and Tim understand that this nation must continue to be a place of possibilities, not just for the few of us, but for all of us,” Mr. Biden told the crowd in a speech that ran well past prime time on the East Coast. Other speakers ran so long that organizers of the event were forced to cancel prepared videos and a musical number to keep the night from running even later.

“Because of the raucous applause interrupting speaker after speaker, we ultimately skipped elements of our program to ensure we could get to President Biden as quickly as possible so that he could speak directly to the American people,” said Emily Soong, the press secretary for the convention.

After his speech, Ms. Harris gave the president a hug. It was clear from the close-up image of the pair that she told him: “I love you so much.”

Mr. Biden’s speech was interrupted numerous times by applause from the crowd, though by the end some delegates and others had already begun to depart. Among those waving signs declaring their love for the president was Representative Nancy Pelosi of California, the former House speaker, whose decades-long relationship was strained when she aggressively pushed for Mr. Biden to step aside following his debate performance.

Asked later by reporters whether he was mad at Ms. Pelosi, the president said: “No. I haven’t spoken to her. No one made my decision but me. No one knew it was coming.”

In his speech, Mr. Biden said he believed that his accomplishments provided 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 achieved more of consequence than any president since Franklin D. Roosevelt. And he vowed to continue working to advance his agenda with the time remaining.

“I’ve got five months left in my presidency,” Mr. Biden said, suggesting how difficult it was for him to abandon his re-election bid. “I’ve got a lot left to do.”

In recent weeks, Democrats have praised Mr. Biden for what they describe as a selfless political act by stepping aside for Ms. Harris. But the president abandoned his desire for a second term in the White House only after weeks of enormous pressure from donors, voters, activists and lawmakers.

“I love the job,” he said on Monday night, echoing a line he used in his address to the nation days after he announced his decision last month. “But I love my country more.”

Mr. Biden’s speech, which had a place of honor as the final one of the night, began at around 11:30 p.m. Eastern time and stretched past midnight. Mr. Biden and Jill Biden, the first lady, immediately left for California to start a two-week vacation that will keep him out of the spotlight as Ms. Harris formally begins the sprint to Election Day.

The president addressed a convention hall filled with delegates who had been among his strongest supporters when he was a candidate. Organizers decorated the hall with digital banners displaying common Bidenisms like “Spread the Faith” in preparation for the outpouring of affection for the 81-year-old president as he departed the stage.

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

This week’s convention — now built around Ms. Harris instead — is being run by the campaign machine that Mr. Biden assembled over the past year, and largely handed over to the vice president when he backed out of the race and endorsed her.

In his comments to reporters as he boarded Air Force One at O’Hare airport in Chicago, Mr. Biden was asked about the enthusiastic reception he received from his fellow Democrats.

“It meant a lot to me,” he said. “It was very nice.”