Exactly one year before Election Day – on November 8, 2015 – Bernie Sanders was asked whether his agreement with Hillary Clinton on basic issues outweighed the conflicts that he proclaimed at every campaign appearance.

Speaking on television rather than on the stump, the Vermont senator conceded reluctantly that he and Clinton concur on some issues. But then he added an entirely gratuitous endorsement:

“And by the way, on her worst day, Hillary Clinton will be an infinitely better candidate and President than the Republican candidate on his bestday.”

That moment of reassuring reason is worth remembering as the debate becomes more rancorous. Clinton isn’t foreordained to win the Democratic nomination, so Sanders neither will nor should hesitate to emphasize their differences.

So far, in fact, his challenge has improved both her candidacy and the national discourse. It is healthy for Democrats to argue about the best ways to ensure more good jobs, higher wages, universal health care, affordable higher education, paid family leave, immigration reform, national security, and a clean energy future.

But the overwrought reaction of some Sanders supporters – who already insist they cannot imagine voting for Clinton because of her campaign donors, her paid speeches, her vote on Iraq, or her support for some of her husband’s policies two decades ago – evokes bad memories of another, truly disastrous presidential campaign.

It is no accident, as they say, that those who “feel the Bern” today include prominent supporters of Ralph Nader’s independent presidential campaign in 2000. Their urge to overthrow the mundane, demand the utopian, reject grubby compromise, and assert moral purity is as powerful today as four cycles ago; and perhaps even more so, especially among those who feel somehow “disappointed” by President Obama. But political decisions based solely on such emotional considerations can prove terribly costly to our country and the world – as we discovered when George W. Bush usurped Al Gore.

Nader and his supporters were not responsible alone for the appalling outcome of the 2000 election but – along with the Supreme Court majority and Gore himself — they bear a substantial share of blame. Their defamatory descriptions of the Democratic nominee were echoed across the media by reporters, columnists, and commentators who knew better — from the pages of the New York Times and the Washington Post to the cable networks.

Mocking Gore for his supposed personal flaws, such as an alleged propensity to exaggerate his achievements, was as fashionable in media and political circles then as it is to denigrate Hillary Clinton now. Clever people delighted in contrasting Nader’s — and even Bush’s! — “authenticity” with Gore’s stiff insincerity. (Indeed, many of the same pundits are still doing versions of the same stupid pundit tricks.)

Besides, according to the Naderites, there were no important differences between the Democratic nominee and his Republican opponent. Or at least none that merited as much concern as Gore’s earth-toned suits and the preppy character he did or didn’t inspire in a romance novel. A wave of such idiotic babble delivered America and the world into a catastrophic Bush presidency.

Fortunately, the parallels only go so far. Sanders chose a far more responsible route than Nader when he decided to run for the Democratic nomination rather than jump to a third-party line. He has focused on substantive issues and admirably dismissed fake scandals like Benghazi and Clinton’s emails. But by repeating his unfounded insinuation that Clinton’s paid speeches and Wall Street donors have somehow corrupted her, he is inflicting damage that will be very hard to mend.

Looking toward the likelihood that either Clinton or Sanders will face Donald Trump next fall, those corrosive tactics are shortsighted. Should Sanders win the nomination, he will want and need Clinton’s support. And should she defeat him, he will and should want her to win — if he believes what he said last November, and understands the exceptionally dangerous threat posed by a Trump presidency.

The next round of Democratic primaries could encourage still more destructive bashing, from either camp or both. The candidates and their supporters ought to think beyond the moment – and pay attention to filmmaker Michael Moore, an outspoken Sanders backer.

On the evening when his candidate won an upset victory in the Michigan primary, Moore tweeted this message: “A special congrats to Hillary for her victory in Mississippi on International Women’s Day. If you win the nomination, we will be there [with] you.”

Once a zealous backer of Nader, Moore eventually apologized for that tragic mistake. Evidently he would rather not feel that sorry again.