President Biden’s decision to drop his bid for a second term will be widely attributed to his age and infirmities. We’re skeptical of that. Not that we’re blind. Everyone can see the difficulties he faced. Our reckoning of the failure of Mr. Biden’s effort for reelection, though, is that he had already moved too left for the American voters, on both domestic and foreign affairs. At the end of the fight, he had no way but to come in from the cold.

This came into focus, at least for us, with the occasional glimpses of President Trump testing unity themes — including an outreach to minorities, a tempering of his policy approach to abortion, and his putting a major labor leader on the GOP stage at Milwaukee. This culminated in a memorable nominating convention, where he reached across the generations for a running mate, and then lit out for the Blue Wall of Michigan, where he’s up by seven points.

While Mr. Biden was trying to decide, his political opponent was boxing him in. Not only tactically but strategically, much of it at odds with recent GOP boilerplate. The new Republican Party is, it suddenly seems, all for tariffs, critical of big corporations (per President Trump’s pals at the Teamsters), and for a new kind of détente with Russia and China. Neoconservative hawks are nowhere to be seen. Where was the opening for Joe Biden?

The first word from Mr. Biden that he was quitting made no mention of the vice president who had, in the 2020 primaries, suggested Mr. Biden was a racist. He quickly released a letter supporting her for president — though she is more to the left than Mr. Biden is. She suggested in an interview with the Nation that she empathizes with antisemitic campus protesters. She shares in Mr. Biden’s failures on immigration, economic, and foreign policy.

We don’t want to be too reflexively negative about Ms. Harris (and we understand other contenders are champing at the bit). There are those who think, too, that if Mr. Biden is too infirm for a second term, the better part of valor would be for him to step down now. We don’t see the logic, though we were against President Nixon stepping down. It’s not clear to us that a president ought to be allowed to quit.

One moment to watch will be on Wednesday, when Prime Minister Netanyahu addresses a joint meeting of Congress. The logic would be for Ms. Harris, in her capacity as president of the Senate, to be sitting just behind the Israeli leader. When Mr. Netanyahu spoke in 2015, Mr. Biden skipped the event. If Ms. Harris attends, it could be a signal she is ready for what in President Clinton’s day was called a “Sister Souljah Moment.”

Mr. Biden himself will be meeting with Mr. Netanyahu on Tuesday. It would be a chance to signal an end of the appeasement the Democrats have favored toward Hamas and Iran. Will Ms. Harris be included now that Mr. Biden could be handing the baton over to her? Our point is that Ms. Harris, a product of the People’s Republic of California, has never had to compete for the center. So, lose or win, this is a hinge moment in her career.

As it is for the Democratic Party. We were against Mr. Biden quitting. It will look to millions as a disenfranchisement of those Democrats who voted in primaries to give him a second term. Our Dean Karayanis made this point nicely, asking in his latest column: “Is this what democracy looks like?” Are these, in any event, the circumstances in which the Democrats want to pivot to a new generation? Who — and where — are their J.D. Vances?

Which brings us back to the beginning of this editorial. President Trump may be trying to suppress his inner demons and present a unifying campaign. At least he and Senator Vance went out from Milwaukee with something of a national platform. The problem the Democrats face is the one Mr. Biden bequeathed to them — the failure to turn away from the sirens of the left and to build a bridge to the center. It’s a bit late to be starting now.