President Biden plans to deliver a prime-time farewell address to the nation on Wednesday, putting a capstone on his five-decade political career just days before he leaves an office he has long revered and is leaving only reluctantly.

The White House would not disclose what Mr. Biden plans to say in his speech, set for 8 p.m. Eastern. But in his final months he has been seeking to cement a legacy as a transformative president that stabilized domestic politics while bolstering America’s leadership abroad, one who ushered the nation out of a pandemic, made historic investments in infrastructure and clean energy, and worked to strengthen democratic institutions both nationally and globally.

Whatever image the president seeks to project, it is set against a backdrop in which he is leaving office deeply unpopular and handing the reins to a successor, Donald J. Trump, whom he disdains and has repeatedly said is unfit to hold power.

Even the location of the speech, from behind the Resolute Desk in the Oval Office, is a reminder that Mr. Biden is not departing as he may have wanted. His last prime-time address delivered there was the 11 minutes he spent in July explaining why he dropped out of the presidential race under pressure from his own party as questions mounted about his age and fitness for another term.

Since Mr. Biden left the race and especially since Mr. Trump’s election victory in November, the president has struggled to maintain the spotlight.

“Farewell addresses are challenging because they aim to put the capstone on an era at a time when most of the country has already moved on to the next one,” said Robert Schlesinger, the author of the book “White House Ghosts: Presidents and Their Speechwriters.”