The time is coming when Bernie Sanders should declare victory -- not because he is going to be the Democratic presidential nominee, but because he has already won so much.
Of course, Sanders knows very well that he cannot wrest the nomination from Hillary Clinton. He lags well behind her in pledged delegates, superdelegates and the popular vote, where he trails by well over three million.
Nobody should be surprised that he couldn't beat Clinton, whose political durability is routinely underestimated by hostile media coverage.
What did seem surprising, however briefly, was the mere possibility that a self-described democratic socialist from a tiny New England state could win the nomination of a party he had never condescended to join.
Even more astonishing is how much this rumpled, sometimes cranky, and formerly obscure politician has achieved during his meteoric flight to fame.
Sanders has proved a concept that many on the left have always cherished: Social democratic ideas, given a fair hearing, can appeal to a much broader segment of the American public than most political scientists ever imagined.
No doubt most voters would still shun "socialism," but millions this year have embraced social democracy, European style, with its emphasis on economic security, worker rights, environmental quality and gender equality.
He has pushed both Clinton, the presumptive Democratic nominee, and her party well to the left of where they were when he entered the race.
Although she can point to much evidence of her own progressive inclinations, his challenge has provoked her to speak up forcefully on income inequality, paid family leave, infrastructure spending and financial reform.
Substantive differences remain between them, but their disagreements are narrow compared with the gulf between the two parties -- or between them and the likely Republican nominee.
And he has led a remarkable mobilization of young activists, from every background, now widely seen as representing the future of the Democratic Party. If they remain active, there will be senators, representatives and perhaps even a president who remember Bernie as their inspiration.
For now, as an "independent" sitting in the Senate Democratic caucus, Sanders can still look ahead to a very productive future. But he must choose a way forward that advances rather than squanders this year's achievements. Already he has taken several steps in the wrong direction.
The relentless personal assault he mounted against Clinton has contradicted his proud assertion that "I've never run a negative ad in my life." Over the past few months he has spent millions of dollars on harshly negative advertising, which has caused real damage to her.
Now he seems to be contemplating a strategy that blatantly violates his own democratic instincts, by persuading superdelegates to switch their allegiance to him.
This doomsday scheme would be troubling even if Sanders' supporters hadn't gathered nearly half a million petition signatures already, demanding that the superdelegates support the candidate with the most pledged delegates and highest vote total. To pursue it would deepen party divisions and forfeit any claim to the moral high ground.
That doesn't mean Sanders ought to quit, not until he has seized every last opportunity to deliver his message. As he continues, however, he must consider carefully what path best serves him, his movement and his country.
More than a few of his angry supporters sound as if they intend to punish Hillary Clinton by refusing to vote for her in November, even against Donald Trump. They seem to hope that Sanders will withhold his full support from her, too. They evidently don't realize that Clinton herself will be fine either way.
But a Democratic defeat would badly injure millions of other Americans""and losing to the Republicans would permanently diminish Sanders, too.
If the Democrats can mobilize enough voters for a big victory, their party may well regain control of the Senate. That shift would give Sanders the chairmanship of the Senate Budget Committee, with substantial influence over taxes, spending and the fiscal priorities of the next White House. His new position would amplify that now familiar voice, speaking up on the issues that matter most to him. And as the new administration begins, he would have in hand the necessary tools to hold Clinton to her progressive campaign promises.
Yet if the Democrats lose because the Vermont Senator and his supporters refuse to unite with Clinton, he will remain muted in the minority -- and his uplifting campaign will be seen as the prelude to a national disaster.
This is not a hard choice.
Joe Conason is a nationally syndicated columnist.