When the crowd members in the United Center first chanted, “Thank you, Joe! Thank you, Joe!” on Monday night, President Biden looked down, fought back tears and soaked in the admiration.

But he knew. He might not have wanted to admit it. But he knew. They were thanking him, yes, for what he accomplished during a lifetime in public service. But they were also thanking him, let’s be honest, for not running again.

It is hard to think of a more bittersweet moment for a president who spent more than a half-century on the stage only now to be involuntarily shown the exit. The warm bath of affection in Chicago, real as it may have been, could go just so far to salve the wounds of the past few weeks.

As much as they cheered Mr. Biden and waved their preprinted “We ♥ Joe” signs, the thousands of Democrats gathered for their quadrennial national convention were sending him off to the presidential retirement home four years before he was ready. Mr. Biden found himself demoted from speaking as the presidential nominee on Thursday night, when as recently as a month ago he had expected to address the convention, to Monday night, an evening usually reserved for the party’s past stars.

Mr. Biden, 81, gave little indication that he was ready to go. While he made a couple of self-deprecating jokes about his age, he barely alluded to his decision to step aside under pressure from fellow Democrats worried that the struggles of the oldest president in the nation’s history would sink the party. When he did, he simply framed it as an act of sacrifice to save American democracy from former President Donald J. Trump.

“It’s been the honor of my lifetime to serve as your president,” he said in a 52-minute speech capping the first night of a convention celebrating the nomination of Vice President Kamala Harris to be his successor. “I love the job, but I love my country more. I love my country more. And all this talk about how I’m angry at all the people who said I should step down — it’s not true.”