Former President Barack Obama, who recovered from a disastrous debate performance to win re-election in 2012, publicly backed President Biden after his poor performance in Thursday’s debate against former President Donald J. Trump.

Mr. Obama also sought to allay concerns among Democrats about keeping Mr. Biden as their presidential nominee.

“Bad debate nights happen,” he said in a social media post. “Trust me, I know. But this election is still a choice between someone who has fought for ordinary folks his entire life and someone who only cares about himself.”

Mr. Obama continued, linking to Mr. Biden’s campaign website, “Last night didn’t change that, and it’s why so much is at stake in November.”

Mr. Obama, who served as president for eight years with Mr. Biden as his vice president, played a significant role in unifying the party behind Mr. Biden during the Democratic presidential primaries in 2020. Now, fearing the consequences should Mr. Biden lose to Mr. Trump in November, he has advised Mr. Biden’s team to help the president get re-elected.

The circumstances are somewhat different, but Mr. Obama faced a similar crisis of confidence in his re-election campaign in 2012. Underestimating his Republican opponent, Mitt Romney, Mr. Obama turned in a performance at the first presidential debate that was widely panned — with just a month to go before Election Day.

The episode set off a reassessment of Mr. Obama’s debate strategy. He returned to confront Mr. Romney twice on a more combative and assertive footing, settling a nervousness that had grown within the Democratic Party after he was caught off guard in his first outing against Mr. Romney.

Mr. Obama was not the only former president throwing his support behind Mr. Biden on Friday. In a statement, former President Bill Clinton vouched for the president, saying on social media that Mr. Biden had given the United States three years of “solid leadership.”