President Biden, in what will be remembered as one of the most vital speeches of his presidency tonight, warned Americans to choose “democracy” over “autocracy,” and gave a full-throated endorsement of Vice President Harris to carry the torch.

Mr. Biden said that while he believed his accomplishments “merited a second term,” he knew that his cause would be carried on by a younger generation in his own vice president. 

The president made the decision to step aside as the presumptive Democratic nominee and forego a second run at the presidency after a number of his own party members called on him to stand down. His disastrous debate performance with Trump in June put on display for the entire country and the entire world his own apparent mental and physical incapabilities. 

In his Oval Office address, he explained that he would step aside “in the defense of democracy,” giving a new generation the chance to defeat the former president — whom he didn’t name throughout his address — a challenge Mr. Biden was increasingly unable to meet. 

“This sacred task of protecting our union is not about me — it’s about you,” he said. “It’s about ‘We the People.’” He warned that America is at a “rare moment in history” where “freedom, justice, and democracy” are on the line. 

“We are a great nation because we are a good people,” Mr. Biden said. 

When the address began, it was clear Mr. Biden — in his complexion, facial expressions, and tone — has aged rapidly even since assuming the presidency less than four years ago, when many Democrats still had concerns about his age. He made it clear in his speech that he no longer felt he had the confidence of his party to lead. 

“In recent weeks, it’s become clear to me I need to unite my party,” Mr. Biden admitted. “Nothing — nothing — can come in the way of saving our democracy. That includes personal ambition.”

He sung the praises of his own administration, including making inroads in combating Communist China’s influence, lowering the price of prescription drugs, investing in infrastructure, and signing the first major gun control legislation in three decades.

On the issues, Mr. Biden was still not trusted by voters to lead going forward, with many telling pollsters that Trump would be better on the economy, affordability, immigration, and crime — some of the top issues for voters this year. 

“We’ve come so far since my inauguration,” the president declared, saying that he was looking forward to doing more work before he leaves office in January. 

“In just a few months, the American people will choose the course of America’s future,” Mr. Biden said, calling on all to make the choice between democracy and autocracy. 

The president has already anointed his successor who he says is able to finish that job and carry the torch. Just since announcing her candidacy and winning her boss’s endorsement Sunday, Ms. Harris has raised more than $120 million, signed up tens of thousands to volunteer for the campaign, and unofficially wrapped up the nomination after the requisite number of delegates announced their support.  

“She’s experienced, she’s tough, she’s capable. … Now, the choice is up to you,” Mr. Biden said. “Whether we keep our republic is in your hands.”

At the end of his address, he thanked voters for giving him the opportunity to serve in public life for more than five decades. “I gave my heart and my soul to our nation, like so many others,” the president said. “I hope you have some idea about how grateful I am. … Let’s act together and preserve our democracy.”