If anyone wondered whether the Bernie ardor had cooled in the last four years, that question was quickly put to rest. A $6 million haul in the first 24 hours is a staggering number—a general election-type number.

Bernie Sanders' announcement that Faiz Shakir would be his campaign manager, a story The Daily Beast broke Tuesday, is another positive sign. Shakir worked in Democratic politics for a long time, before he left to do a bang-up job at the ACLU. His presence should make for a campaign that, while no less committed to the Sanders platform than in 2016, may be less consciously opposed to the party whose nomination he seeks.

This is the key thing, for the sake of the party, the country, and the defeat of Donald Trump. Sanders really should do one thing differently this time around. Well, two things. First, he should join the Democratic Party. I know the fact that he’s not a Democrat is a big draw for a lot of people, and therefore I know very well that he’s not going to do it. But as much as many of his followers love it, it cheeses some other people off. You want to be the head Democrat? Well, become a Democrat. But I suppose he won’t, so I won’t waste any more space on that.

But here’s the second thing, and it’s more real. He really has to communicate to his followers this time around that it isn’t Bernie or Bust. No one can seriously dispute that he failed to do that in 2016 in any number of ways. He attacked Hillary Clinton too much, too pointedly, and for too long. And there’s no question that his attacks hurt her in the general election and helped make Donald Trump president. Yes, she lost, not him; I’m not blaming Bernie. But his attacks hurt.

I won’t go over it all, because I’m committed to not relitigating 2016. I wrote last month that I was going to try hard this time around not to get too caught up in taking sides in the primary, and that means not using this particular column to angrily catalogue 15 things Bernie did that hurt Hillary. But everyone knows. Well after a Sanders win was mathematically impossible, he kept at her, up to the convention. Even his stock compliment to her was backhanded: “On her worst day,” he would say, Hillary Clinton is 50 or 100 times better than Donald Trump. It sounded like praise, but it subliminally reminded everyone listening that Clinton had, and would continue to have, worst days. He did come around in the fall to being supportive, but a lot of Clinton people thought that was too little too late.

But let’s talk about 2020. No, better still, let’s talk about 2022.

Imagine this: The Democratic primary of 2020 tore the party to pieces, and Donald Trump, all but under indictment, somehow eked out another Electoral College victory. The Republicans held the Senate, and the Democrats the House, but impeachment is basically off the table. Ruth Bader Ginsburg has finally given up and retired, as has Clarence Thomas, so two more under-50 extremely right-wing justices are on the Supreme Court. It’s now a 6-3 conservative court and will stay that way for a couple more decades at least. In the executive branch, labor and regulatory law will be eviscerated. Nothing will be done to address climate change. And on and on and on.

You discount this scenario at your peril. I get the argument that Trump should lose to any competent Democratic nominee. But I get the counter-argument all too clearly, too. The only way Trump can win is by convincing millions of people that the Democrat is just unacceptable under any circumstances. This will involve a campaign of horrendous lies and smears against whoever is the nominee. He will catch the scent of that nominee’s weakness, and he will hammer at it and hammer at it, hoping to scare tremulous and confused voters into voting for him.

Given that reality, there is really only one terrible and unforgivable thing the Democratic contenders can do to one another, and that is to use the primary season to expose that Achilles Heel and worsen it. This is what Sanders did with respect to Clinton in 2016. By calling her corrupt and in hock to Wall Street, he set up Trump’s “Crooked Hillary” line of attack.

This kind of thing cannot happen again. Democrats (and Sanders) will go after one another, but they have to limit their attacks to matters that won’t resonate in a general election campaign—things that matter mainly to primary voters. Laying the groundwork for Trump’s general election attack campaign—in any way, shape, or form—will be unacceptable.

That intensity leads many of his backers to the Bernie or Bust view of things. And it’s incumbent upon him to say to them, “No, let’s not have that again; of course I want the nomination, but if the voters decide otherwise, they decide otherwise, and the important thing is to stay together and not hand Trump ammunition.”

His money haul shows that the conventional wisdom that he’ll be weaker this time because there are other progressive alternatives is overblown. There’s much to respect in a person being able to build that kind of movement and attract that kind of loyalty, as well as in the fact he’s never wavered from his platform and the Democrats have come to him. If it makes him the nominee, all Democrats need to get behind him.

But if it doesn’t, his responsibility is to lead his supporters toward making a quick and ungrudging peace with the party and its nominee. He is a (if not the) frontrunner, and this is a burden frontrunners carry. Last time, no one thought the worst could happen. This time, as we’re reminded hourly, we know it can.