NEW YORK — Mr Bernie Sanders has had a stunning impact this year, helping set the political agenda and winning the passionate embrace of a demographic a quarter his age. A socialist, Jewish, non-pandering candidate who didn’t kiss babies but lectured their parents on social justice won 22 states. But now he has lost. It’s time for him and his followers to stop sniping and start uniting.

Mr Sanders has said he will ultimately support the Democratic ticket, and I’m sure he intends to. But for now he’s still dividing more than coalescing.

In a New York Times/CBS News poll last month, nearly one-fourth of Sanders supporters said that in a Hillary Clinton-Donald Trump matchup, they would either vote for Trump (which suggests bipolar disorder!) or stay home. That figure is inflated by bitterness and resentment, but if some Sandernistas sit on their hands this fall they could help elect a man antithetical to everything they stand for.

At this point, Mr Sanders has essentially zero chance of becoming our next president. Meanwhile, there is a modest risk that continued Democratic warfare will cost Ms Clinton the election. The upshot is that continuing to tilt at windmills is many, many times more likely to elect Mr Trump than Mr Sanders.

We’ve seen this before. In 1968, liberal disenchantment with the Democratic nominee, Mr Hubert Humphrey, assisted in the election of Richard Nixon. In 1980, Mr Edward Kennedy’s endless challenge to Mr Jimmy Carter undermined Mr Carter and probably gave Mr Ronald Reagan a lift.

And in 2000, many liberals regarded Mr Al Gore the way some see Clinton today, as a flip-flopper short on inspiration and convictions. So a small number voted for a third-party candidate, Mr Ralph Nader, probably helping put Mr George W Bush in office.

Mr Nader, whom I admire for his transformational impact on consumer rights, disagrees: He tells me that it’s absurd to blame him for Bush’s election and that he wants Mr Sanders to continue his campaign.

“Why would he want to lose his bargaining power?” Mr Nader asks, suggesting that by staying in the race, Mr Sanders can influence the Democratic platform and Ms Clinton’s choice of a running mate. Anyway, he says, “Trump’s going to implode.”

He’s probably right on that count. I would bet that Mr Trump will lose, and I’d even give 2-to-1 odds. But I remember how my mother in 1980, as a fan of Mr Carter, was overjoyed when Reagan became the Republican nominee since she figured that assured Mr Carter’s re-election. She wasn’t so happy a few months later.

Presidential campaigns are driven in part by surprises: What if there is a new wave of Central American refugees or a terror attack by a Muslim recently admitted to the US? Either would bolster Mr Trump’s chances.

The success of both Mr Trump and Mr Sanders this year should inspire humility on the part of all of us about predicting election results. I agree with Mr Nader that it’s almost unthinkable for Mr Trump to be elected. Then again, it once was unthinkable that he would win the Republican nomination.

Mr Sanders supporters should also remember that they agree at least in part with Ms Clinton on Wall Street excesses, income inequality and college debt. Likewise, whatever their distaste for the Clintons, they probably share her views on reproductive health, on Supreme Court nominees, on inclusiveness toward Muslims and Mexican-Americans, on immigration reform, on early-childhood investments, on a stronger social safety net, on women’s rights around the world, on reducing mass incarceration and on a global pact to confront climate change.

Sen. Jeff Merkley, D-Ore., who has been the only senator to back Sanders, acknowledges that now “we have a nominee”. He tells me that Mr Sanders will continue his primary race through the Washington, DC, vote next week but ultimately will focus on party unity.

“When I talked to Bernie when he was first thinking about running, he made it absolutely clear that he didn’t want to do anything that would result in the journey that we experienced with Ralph Nader,” Mr Merkley said. “He will do everything possible to make sure that Trump is not in the Oval Office, and to do *everything possible’ certainly means that we’ve got to come together not just as a formality but in an inclusive, emphatic, unified fashion.”

In 2008, at about this time, Ms Clinton stepped up and gave a powerful endorsement of Mr Barack Obama. But she and Mr Obama agreed on almost everything, while Mr Sanders disagrees with Ms Clinton on some issues and still exudes scorn for the Clinton campaign.

“Our struggle continues,” Mr Sanders said in a new fundraising email on Wednesday. Speaking in California on Tuesday evening, he did little to discourage his audience as it booed mention of Clinton.

That’s just irresponsible. And now that Ms Clinton has won a majority of pledged delegates, it’s a violation of Mr Sanders’ own principles to try to get superdelegates to vote for him rather than for the people’s choice.

“Defying history is what this campaign has been about,” Mr Sanders said Tuesday, but at this point he’s also defying his own values — and, just maybe, bolstering the prospects of the candidate who is the anti-Sanders.

I understand the passion and heartache of his followers, but I watched such idealism help elect Nixon and George W Bush, and I flinch at the thought of similar idealists this year helping to elect a President Trump. THE NEW YORK TIMES