Donald Trump has proven to be an important threat to democracy. He is erratic, self-interested and unworthy of public trust. He tried to undermine the integrity and fairness of elections. His supporters have publicly described a 2025 Agenda that would allow him to fulfill the most extreme promises and threats. He has promised to be a new kind of president if he’s re-elected, free from the constraints imposed by the American political system.

Mr. Biden said that the best candidate to defeat this threat is him. His argument is based largely on his victory over Mr. Trump in the 2020 election. This is no longer enough to justify Mr. Biden as the Democratic nominee for this year.

The president had to convince the American people that he could handle the responsibilities of the office for which he wants to run again. But voters cannot ignore the obvious: Biden has changed since four years ago.

On Thursday night , the president was portrayed as a shadow of an outstanding public servant. He struggled with explaining what he’d accomplish in a new term. He struggled with how to respond to the provocations of Mr. Trump. He struggled to hold Trump accountable for his lies and failures, as well as his chilling plans. He struggled more than once to finish a sentence.

Mr. Biden was an admirable President. The nation prospered under his leadership. The greatest public service that Mr. Biden could do now is to announce he won’t run for reelection.