Welcome to FiveThirtyEight’s weekly politics chat. The transcript below has been lightly edited.

sarahf (Sarah Frostenson, politics editor): This is usually the point in the primary season where the field has started to winnow — candidates who’ve had poor performances in Iowa or New Hampshire drop out. But no such luck this year. The debate stage tomorrow is actually getting larger, not smaller, if you can believe it.

But even though the field remains crowded, there’s an increasingly clear front-runner: Sen. Bernie Sanders. Sanders won, and he won pretty handily — he won the final, realigned popular vote by 21.6 points, so Sanders’s odds are pretty good in our model: He’s got a 45 percent shot of winning the majority of pledged delegates.

[Our Latest Forecast: Who Will Win The 2020 Democratic Primary?]

However, South Carolina, which is where the primary moves next, could be a bit of a curveball for Sanders, as it’s Biden — not Sanders — who is in the lead there. Things are really close, though, so it’s probably best thought of as a tie. But it also means the stakes at tomorrow’s debate are pretty high. Biden is banking on a strong performance and winning South Carolina, while other candidates like former Mayor of South Bend, Indiana, Pete Buttigieg, Sen. Elizabeth Warren and Sen. Amy Klobuchar need to demonstrate that they’re can build diverse coalitions, something they struggled to do in Nevada.

So what are you looking for going into tonight? Will the candidates finally attack Sanders as the front-runner?

perry (Perry Bacon Jr., senior writer): Buttigieg’s post-Nevada speech was basically a series of attacks on Sanders. Biden’s had some elements of that, too. Former Mayor of New York Michael Bloomberg is coming after Sanders. Tom Steyer, too. Yes, I think this debate will have several people going hard after Sanders, looking really toward Super Tuesday.

natesilver (Nate Silver, editor in chief): I think this whole week is gonna be CAN ANYONE STOP BERNIE SANDERS?!?!?!?!?!?!? week and that theme is likely to feature heavily at the debate.

The thing about Sanders is that he’s a pretty steady (if not always spectacular) debater and not the easiest guy to knock off-kilter.

And if it seems like everyone is attacking him, well, that plays into his message in certain ways.

sarahf: Yeah, and to Perry’s point, Buttigieg really didn’t mince words on Saturday when he told supporters that Sanders is waging an “inflexible ideological revolution that leaves out most Democrats.” But I’m struggling to wrap my head around how accurate that claim is. Buttigieg came in third in Nevada — significantly behind Sanders. So is Sanders’s “revolution” really leaving out most Democrats? Seems like a fair share of Democrats have been happy to vote for Sanders so far!

Buttigieg and the other moderates need to come up with something better, no?

geoffrey.skelley (Geoffrey Skelley, elections analyst): It’s possible they need something better, but things might look different if a couple of the more moderate candidates got out of the way. Sanders might still lead the field — recent national polls suggest he would — but if a few folks dropped out, it would make it a lot clearer who the alternative to Sanders is. At the moment, there isn’t really one obvious answer to that question. Yet, there’s plenty of evidence that points to the idea that many Democrats might not want Sanders as the nominee. After all, Sanders only won between 25 to 34 percent of the initial popular vote in Iowa and Nevada (so before realignment), and just 26 percent overall in New Hampshire — not exactly an overwhelming share of support.

natesilver: I don’t know … a lot of Democrats do want a more moderate nominee, at least in the abstract. And many associate moderation with electability.

Calling Bernie out as being uncompromising is maybe not a bad line of attack either, since Democrats tend to like compromise.

perry: Ten days of sustained attacks on Sanders could dip his numbers. It would help if the other candidates maybe moved away from Medicare for All, which seems likely it’s decently popular among Democratic primary voters, and found ways to attack him on different issues, though.

natesilver: I do think there’s something about sustained vs. sporadic attacks that matters here.

Like, few things in politics are truly new. Bernie has been around for a while. But there’s always been some shinier object that was rising in the polls — Harris! Warren! Buttigieg! Bloomberg! — that was always more the center of attention, but now that’s changed with Sanders being truly at the center.

perry: Also, a big part of the next 10 days or so is the other candidates working to keep Sanders in the 25-to-33 percent range. But if Democratic voters start falling in line and accepting Sanders as the inevitable nominee, and he gets closer to 40 percent in some states, that would be bad for the others.

natesilver: Yeah, it sorta feels like part of what candidates may be hoping to do is to prevent him from rising further, rather than actually lowering his numbers.

Because he’s likely to get a bounce from Nevada, so even if he has a mediocre debate/the scrutiny hurts him a bit, it might just cancel that out — not actually make his numbers negative relative to where he stood, pre-Nevada.

geoffrey.skelley: I do wonder if the fact that the stage will be even more crowded plays into that to some extent, Nate. Because Steyer has made this debate, there’ll be seven participants this time around, so if you’re trying to stand out as the No.1 alternative to Sanders, that might be even harder. Especially because I assume Bloomberg will still get a lot of attention, even though his standing may have taken a hit after the last debate.

natesilver: It’s kind of a big moment for Sanders, Bloomberg AND Biden, since Biden clearly needs to win South Carolina.

But I do think we’re at the point now where it’s in the mutual interest of every other candidate for Sanders to struggle. There are no longer tactical considerations that outweigh that.

If Sanders gets say, 26 percent of the vote on Super Tuesday instead of 34 percent, that’s a big deal and makes it much more likely that someone else wins.

perry: What was useful last week, in attacking Bloomberg, was that the candidates said interesting stuff — like Warren demanding on stage that Bloomberg release women who were former employees of his company from non-disclosure agreements. So a smart approach from at least one of the non-Sanders candidates would be to attack him in a way that goes beyond questions of electability and Medicare For All, etc.

sarahf: Is it enough for Biden to win South Carolina at this point if the margin between him and Sanders is really close, though? Or does Biden really need it to be as decisive as, say, Nevada was for Sanders?

natesilver: Our model says that the margin matters at the margins, but it’s mostly winning that counts.

Now, if Biden wins by 20 points, maybe we’re in a situation where “it’s not clear who’s the favorite anymore” vs. if he wins by 2 points, the story is, “at least this is still interesting, but Sanders is still in the best position.”

If he LOSES by 2 points, though, it’s hard to see his campaign recovering, and it puts Sanders in a very, very good position.

perry: It would be ideal for Biden to win South Carolina and be the clear second-place finisher on Super Tuesday, so as to flush out the other candidates and get it down to a two-person race. But if come next Wednesday, Biden has won South Carolina and Alabama (two states that vote in the next 10 days that have large black Democratic electorates) but finished say, third or worse in California and Texas, that is not ideal for him — or the people who want Sanders to be defeated.

geoffrey.skelley: Obviously, an unexpectedly large margin would be good for Biden, and the media might be quite open to a comeback narrative, but it’s hard to know at this point how strong Biden’s lead is there.

One potential bit of good news for the Biden camp is that longtime South Carolina Rep. Jim Clyburn, who is the highest ranking African American in Congress, may endorse Biden on Wednesday. This endorsement could be huge for Biden and would signal either that more black voters are ready to line up behind Biden, or it’d be enough to push some undecided black voters into Biden’s camp.

perry: Yeah, the CBS/YouGov poll released on Sunday suggested a fairly close race in South Carolina: Biden at 28 percent, Sanders 23 percent, and Steyer 18 percent.

But maybe a big push by black, pro-Biden officials like Clyburn and a sustained week of attacks on Sanders makes it less close. There is no big early voting drive in South Carolina like there was in Nevada, so the events of this week will matter.

natesilver: It’s a little hard to figure out where the conventional wisdom is on South Carolina. Do people see it as a toss-up? Or still as Biden’s state to win?

The polls are closer to a toss-up, although they slightly tilt toward Biden. It’s important to note, though, that we don’t have any post-Nevada polling yet.

I would also note that there’s a tendency for the polls in South Carolina to underrate the candidate who has more black support — namely Obama in 2008 and Clinton in 2016. Anecdotally, there seems to be some evidence that black voters are more likely to tell pollsters they’re undecided, and that seems to be true in South Carolina as well. In the Marist poll, for instance, 13 percent of black voters said they were undecided vs. 5 percent of whites.

geoffrey.skelley: Yeah, Nate, I realize this is anecdotal, but when I was down in South Carolina a week ago at a Biden event at a historically black church, a couple of voters told me they weren’t sure about supporting Biden, but they also didn’t really have another person they were considering.

perry: Sanders released his campaign schedule for this week, and at least as of right now, he is in Virginia on Saturday and South Carolina on Friday. On the other hand, Biden is in South Carolina both days. Sanders was also in Texas for the Nevada caucuses.

His team is super-focused on Super Tuesday, which is smart. But it also tells me that while they want to win South Carolina, they are not necessarily expecting a win, nor are they going to kill themselves to get one.

natesilver: According to the FiveThirtyEight primary forecast, that strategy might be short-sighted, i.e., they’d be better off going for the kill in South Carolina because that makes it so hard for Biden to come back.

But maybe they aren’t super confident about their prospects in South Carolina and want to lower expectations. There’s this tendency to look at African American voters as a monolith when the ones in South Carolina are liable to be far more conservative than the ones in, say, Nevada, among whom Bernie did fairly well.

perry: The other weird thing about South Carolina is Steyer. I tend to think he will decline in the polls as the week goes forward, and that he will be closer to polling in the low teens than the high teens. But I also don’t know who his support would go to. But maybe I’m wrong, and he does really well. He has spent a lot of time in the state, particularly in courting black voters.

sarahf: Where does South Carolina leave the other candidate who aren’t Biden or Sanders, though? As we’ve said, it’s important Biden wins there because he’s made so much of his campaign about his ability to win black voters. But what if Buttigieg, Klobuchar and Warren all finish in the single digits again?

Do they power through to Super Tuesday, even though this will now be the second state where they’ve failed to build strong, diverse coalitions?

natesilver: Our model doesn’t think Buttigieg has much of a path, although it’s worth noting that he’s had pretty good results so far (1st/2nd/3rd or 2nd/2nd/3rd if you prefer) so it seems a little much for people to be asking that he should drop out. His problem is that there aren’t any Midwestern states on Super Tuesday except for Minnesota, where Klobuchar is a big problem for him. But he’s closed strongly relative to his polling averages in most states so, I dunno, I guess he’s just hoping to do so again.

Warren has a — somewhat good! — excuse to continue based on her post-debate polling looking stronger.

I don’t get what Klobuchar is doing. Or Steyer, frankly.

geoffrey.skelley: Warren remains not that far behind Biden and Bloomberg in national polls. Whereas, yeah, Klobuchar is at about 5 percent in our national polling average, and Steyer is at 2 percent. Even if Steyer gets a strong third in South Carolina, I don’t see a path opening up for him in Super Tuesday states.

perry: Buttigieg, Klobuchar and Warren should be in South Carolina some this week to prevent being cast as ignoring black voters. But they should try to do as little as possible there. Super Tuesday just has better opportunities for them than a state in the South where the majority of the electorate is black. Not to mention, Steyer has camped there for weeks.

natesilver: Having Super Tuesday SO close to South Carolina is sort of an important wrinkle in the scheduling.

And probably a flaw, I’d say.

sarahf: Wait, why a flaw, Nate?

natesilver: Because it doesn’t really give voters and the candidates time to react to South Carolina? Or, conversely, maybe you get a fleeting reaction that isn’t durable?

perry: I agree with Nate. The first state with a lot of black voters is scheduled when the candidates also have a lot of incentive to focus on Super Tuesday, and it’s just difficult to manage.

sarahf: It seems as if campaigning in the Super Tuesday states has been really hard across the board, and only Sanders and Bloomberg have done an OK job of it, especially in California and Texas, which are delegate-rich.

natesilver: Well, maybe more advertising than campaigning in Bloomberg’s case. There’s been some of the latter but a lot of the former, obviously.

And if Bloomberg continues to slump in the polls, we could be looking at a case where he finishes just UNDER 15 percent in most places, and therefore, receives few delegates of his own, but nonetheless takes away 13 to 14 percent of the vote from other moderates. That would be very, very, very helpful to Sanders.

Conversely, if Bloomberg gets 20 percent, he would at least win some delegates and lower Sanders’s total, which isn’t as good for Bernie.

geoffrey.skelley: With that in mind, I wonder if we see a lot of Bloomberg bashing again in this debate and maybe not quite as much Sanders bashing as you’d expect. Pushing Bloomberg down probably helps someone like Biden a fair bit, and obviously in the last debate he was the chief target of criticism.

sarahf: That’s a good point, Geoff. But some of what happens next seems to hinge on Biden actually pulling off a strong victory in South Carolina. With the field still so crowded it’s hard for me to understand how the “moderates” mount a credible threat to Sanders at this point, when they’re all still in the process of competing with each other. It seems like the kind of chaos that Sanders mainly benefits from. Is that fair?

natesilver: For what it’s worth, I don’t think whether the chaos benefits Sanders is as clear as people assume.

If you have so much chaos that you wind up with a contested convention, that doesn’t necessarily benefit Sanders!

And he isn’t necessarily an underdog if the race comes down to 1-on-1. He might be a favorite, in fact, in several of the matchups. Would you rather bet on Sanders or Mayor Pete head-to-head, for instance? I’d take Bernie.

You could also wind up with a scenario — if last week’s debate had a big impact — where the three most viable candidates going forward are Sanders, Warren and Biden, which is a “chaotic” scenario that is not so great for Sanders.

geoffrey.skelley: Sanders may be aided by a crowded field in that, many candidates won’t crack the 15 percent delegate mark statewide or in most districts. That means, if say, Sanders wins 30 percent statewide and in most districts while another candidate wins 15 percent exactly, with the rest splitting the vote and finishing under 15 percent, then Sanders ends up with about two-thirds of the delegates that state has to offer. In a place like California, that works out pretty well for him!

perry: I think the lane stuff is both relevant and not perfect. Do I think it would help Biden or Buttigieg if either were the only candidate against Sanders? Probably. But I absolutely do not think that Sanders would keep just his current 25 to 30 percent and Biden would get the other 70 percent. The reasons this field is so fractured and Sanders keeps winning are: 1) Sanders is pretty popular among Democrats, and 2) The center-left candidates all have some flaws (Buttigieg’s weak numbers among black and Latino voters, for example).