President Biden has repeatedly and rightfully described the stakes in this November’s presidential election as nothing less than the future of American democracy.

Donald Trump has proved himself to be a significant jeopardy to that democracy — an erratic and self-interested figure unworthy of the public trust. He systematically attempted to undermine the integrity of elections. His supporters have described, publicly, a 2025 agenda that would give him the power to carry out the most extreme of his promises and threats. If he is returned to office, he has vowed to be a different kind of president, unrestrained by the checks on power built into the American political system.

Mr. Biden has said that he is the candidate with the best chance of taking on this threat of tyranny and defeating it. His argument rests largely on the fact that he beat Mr. Trump in 2020. That is no longer a sufficient rationale for why Mr. Biden should be the Democratic nominee this year.

At Thursday’s debate, the president needed to convince the American public that he was equal to the formidable demands of the office he is seeking to hold for another term. Voters, however, cannot be expected to ignore what was instead plain to see: Mr. Biden is not the man he was four years ago.

The president appeared on Thursday night as the shadow of a great public servant. He struggled to explain what he would accomplish in a second term. He struggled to respond to Mr. Trump’s provocations. He struggled to hold Mr. Trump accountable for his lies, his failures and his chilling plans. More than once, he struggled to make it to the end of a sentence.

As it stands, the president is engaged in a reckless gamble. There are Democratic leaders better equipped to present clear, compelling and energetic alternatives to a second Trump presidency. There is no reason for the party to risk the stability and security of the country by forcing voters to choose between Mr. Trump’s deficiencies and those of Mr. Biden. It’s too big a bet to simply hope Americans will overlook or discount Mr. Biden’s age and infirmity that they see with their own eyes.

If the race comes down to a choice between Mr. Trump and Mr. Biden, the sitting president would be this board’s unequivocal pick. That is how much of a danger Mr. Trump poses. But given that very danger, the stakes for the country and the uneven abilities of Mr. Biden, the United States needs a stronger opponent to the presumptive Republican nominee. To make a call for a new Democratic nominee this late in a campaign is a decision not taken lightly, but it reflects the scale and seriousness of Mr. Trump’s challenge to the values and institutions of this country and the inadequacy of Mr. Biden to confront him.