Meanwhile, as Donald Trump destroys American credibility for the next 20 years (George W. Bush having destroyed it for the last 20; are we seeing any kind of pattern here, folks?), the Democrats gather for Tuesday night's next debate.

You’ll see a lot of segments on cable news for the next 36 hours about what’s going to happen and who needs to do what. Most of it, I’m guessing, will center on how Elizabeth Warren will handle her new front-runner role, and how Bernie Sanders will come across after his heart attack.

I’m going to be watching for something else. I’m going to take note of who violates Tomasky’s Commandment: Thou shalt not attack a fellow Democrat in a way that can be recycled as a trash right-wing talking point in the general election.

In a primary, candidates need to differentiate themselves. And in this field, there are real differences. They can and should critique one another’s ideas and proposals—that’s what primary voters making up their minds need to see.

What we don’t need are attacks that are framed in a way that the right wing can pick up and use next fall. That’s crossing a line. Tuesday night, I’ll be looking in particular for two things. One, how will candidates who don’t back Medicare for All criticize Warren on health care? And two, will anyone fail to defend Joe Biden against Trump and Rudy Giuliani’s groundless attacks?

First, on Warren. Obviously, there are real differences among the candidates on health care. They should of course discuss those differences. And it’s fine for someone to challenge Warren—and Sanders, but hers is the more relevant case since she’s the one who has zoomed up in the polls—on the pledged elimination of private health insurance and what the average family’s M4A tax bill might be. The tax bill question is one that Warren has dodged pretty much completely, while Sanders did acknowledge taxes would go up but has never said by how much. In my view, this looms as a potentially huge general-election point of vulnerability, so it has to be aired.

But there are different ways to do it. What’s the wrong way? It’s admittedly a little subjective, but as Justice Potter Stewart famously said of pornography, we’ll know it when we see it. If you can close your eyes and imagine the speaker’s words a year from now coming out of Trump’s mouth, or Sean Hannity’s, or the scare-narrator’s in a 30-second attack ad, the red line has been crossed.

One can critique M4A while still affirming two inviolate principles: one, that universal coverage, however achieved, is a moral necessity; and two, that health care is a right and not a privilege. Those are things that right-wingers would never say. Critiques of Warren and Sanders couched in those terms are fine. Those that aren’t cross the line.

Now, on Biden. Since the Ukraine story broke big and Trump and Giuliani have warbled on about how corrupt Joe and Hunter Biden supposedly are, I’ve been distressed to see that not all Democratic contenders have leaped to Joe’s defense.

Many have. Cory Booker was most forthright. He issued a strong statement on Oct. 4 that called on TV stations to refuse to air Trump’s ads attacking the Bidens (“garbage”). The day before that, Kamala Harris had tweeted a somewhat less stout defense, addressing Trump and saying that “telling lies about Joe Biden won’t protect you from the truth.” And Beto O’Rouke called Trump’s Biden ad “a disgrace.” And just this Sunday on CNN, Pete Buttigieg defended the Bidens and called Trump a “walking conflict of interest” and turned Jake Tapper’s question around on the Trump family.

The notable exceptions here, of course, are Warren and Sanders. In late September, Warren was asked at a town hall in New Hampshire if she’d let her vice president’s offspring serve on the board of a foreign company, and she said no; but she stopped short of adding any defense of the Bidens, and of course Fox News ran with it. Around the same time, Sanders was asked about Hunter Biden by reporters in Iowa, and he said he didn’t know enough facts. He didn’t denounce, but he didn’t defend.

It’s not necessarily some thought-crime that they said what they said. They were on the stump, it was just as the Ukraine story was breaking. But if it comes up Tuesday, when many millions of people are watching, they—and everyone who’s given a chance to speak on the matter—should affirm with no equivocation that they find Trump’s attacks “garbage” and “lies” and “a disgrace,” too.

I say this not in defense of Biden. He will face a legitimate question about why he didn’t just tell his son, “No—not Ukraine, and not now.” Yes, parents can’t always tell their grown kids what to do. If I want my daughter to go to med school but she wants to join the circus, I can’t force her to go to med school. But this is a little different. Joe Biden was the vice president of the United States. He should have just shut that down so these questions couldn’t even be raised and this innuendo wouldn’t have any oxygen. It’s good that Hunter Biden said what he said Sunday about getting off that Chinese board and not doing this again if his father is elected. I wish he’d thought of that way before. But again, there is zero evidence of any Biden criminality.

Rather, I say this in support of what is to me a core political principle. Democrats have to close ranks around any one of their number who is the target of baseless and dishonest right-wing attacks. For the last two weeks, it happens to have been Biden on the receiving end of the sleaze. But next, it’s going to be Warren. Or it might one day be Sanders or Buttigieg or whomever.

Trump will tell whatever lie pops into his head about any of them and drive it and drive it, forcing the media to cover it. And as we know, it usually takes the media a few days to say, “Hey, no, wait, this is a lie,” by which time at least some damage has been done.

If Democrats don’t close ranks immediately when that happens, they’re sending a very bad signal to those of us who follow all this super closely—that is, we can’t help but suspect that just maybe they don’t really mind that other candidate being torn down, because they stand to benefit from it. Is that what we want: a Democratic contender who seeks to nudgey-winky benefit from Trump’s corrupt immorality?

So that’s what I’ll be watching for Tuesday night. There’s a way of keeping disagreements “in the family,” and there’s a way of planting seeds that will sprout to the right’s benefit next fall. Any candidate who by either words or silence participates in the latter process is “objectively,” as the leftists used to say, helping Trump.