Biden Made a Courageous Choice. Democrats Must Seize the Opportunity.

President Biden’s decision to exit the 2024 presidential election is a fitting coda for a man whose life has been devoted to public service. Mr. Biden has served the nation well as its president. By agreeing to step down when his term ends in January, he is greatly increasing the chance that his party is able to protect the nation from the dangers of returning Donald Trump to the presidency.

Majorities of Americans have consistently said they did not believe Mr. Biden could lead the nation for another term, citing longstanding fears about his age and fitness that have only grown in recent months. Had he remained at the top of the ticket, he would have greatly increased the likelihood of Mr. Trump retaking the presidency and potentially controlling both houses of Congress as well. Mr. Biden himself has consistently warned that specter presents a profound threat to the nation and its democratic traditions.

Mr. Biden has now done what Mr. Trump never will: He has placed the national interest above his own pride and ambition.

Mr. Biden’s departure gives Democrats an opportunity to refocus public attention from questions about the president’s fitness to the manifest moral and temperamental unfitness of Mr. Trump — and to the dangers of rearming him with the considerable powers of the presidency.

Mr. Trump is a felon who flouts the law and the Constitution, an inveterate liar beholden to no higher cause than his self-interest and a reckless policymaker indifferent to the well-being of the American people. His term in office did lasting damage to the people and the project of America and to its reputation around the world. In a second term he would operate with fewer restraints and more willing enablers, and he and his emboldened advisers have made clear they intend to exercise power ruthlessly.

Yet it’s not enough to describe all the harm Mr. Trump would do to this country: The Democratic Party needs to offer the American people a road map to a better future. The new presidential and vice-presidential nominees will represent a fresh chance to remind voters of longstanding differences between the two parties.