It’ll all end in tiers, as our mothers used to (almost) say. At least for the moment, that’s what we have in the polling for the Republican presidential nomination. The polling averages at Real Clear Politics, especially over the last 30 days, show that the race has begun to stratify into three nearly distinct groups:

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Obviously, Donald Trump and Ben Carson occupy the top tier and have a significant amount of distance between themselves and everyone else. Marco Rubio and Ted Cruz have moved into a second tier and a virtual tie, due both to improvements in their own polling and declines in the rest of the field, which form the third tier. Jeb Bush looks stuck between the second and third tiers but is declining steadily, while Carly Fiorina has declined more rapidly into the trailing pack.

With the fourth debate coming shortly, the question will be whether anyone can disrupt this stratification with a debate performance alone. All of these candidates have participated in the three previous debates, and for almost everyone except the frontrunners, it’s a familiar format and familiar environment. The top two tiers only need to keep on keepin’ on, at least for a while, and follow success with success.

For the rest, though, this is probably a do-or-die moment. In my column for The Week, I offer a preview for each tier, splitting off the undercard debate into a separate category that may not last much past tonight either:

The undercard: This is a four-person debate: Christie, Huckabee, Jindal, and Rick Santorum. Frankly, the only candidate in this group with any shot of moving up is one of the men who got bumped from the main stage: Christie. A video of Christie telling a moving story about a friend who died from an addiction to painkillers has gone viral. If Christie can get to that story in the debate, it might be distinctive enough to contrast with the normally contentious tone of the undercard debates. The other candidates have shown no hint of capturing the imagination of voters in three successive debates, and only Jindal shows any life at all in Iowa.

The pack: Carly Fiorina and Rand Paul have been consistent in the debates, but consistency alone won’t help them. Fiorina has had three good debates and got a boost in polling after the second, but it dissipated quickly, and even another good debate performance will likely not lift her a second time. Paul, meanwhile, has seen his two freshman Senate colleagues catapult into the second tier, while he muddles along at 3 percent.

By contrast, John Kasich and Jeb Bush have tried too hard to put on different personas in the debates. Both did damage to themselves while trying to damage others — Kasich aimed and missed at Trump and Carson, while Bush took a muddled swipe at Rubio and got embarrassed by Rubio’s condescending response. Kasich has to find a middle ground between his Happy Chappy and Grumpy Grampa personas in the last two debates, and hope that he can grab a viral moment for something other than a rant. Bush hasn’t learned from his belly flop; his team has telegraphed yet again that they will launch attacks on Rubio rather than Trump or Carson, making Team Bush look more desperate than ever. Fending off attacks from the GOP’s most establishment candidate might end up boosting Rubio, especially if Bush flubs his attacks tonight as badly as he did in the last debate.

The one with the most upside in this scenario is Chris Christie, whose personality will stand out even more with only three others on stage. He has the sense of the electorate, even if he may not be particularly well positioned for it, in that people are looking for someone who can get things accomplished rather than just rant. Huckabee might dilute some of that impact, but I’d expect Christie to get the most attention tonight in the undercard.

Otherwise, there doesn’t seem to be much opportunity for a breakout, not unless either Trump or Carson stumble badly and voters start reassessing the race. They’ve done well through three debates, though, and they’re getting more comfortable with the format. Don’t expect them to take any risks, because … they don’t need to do so. And they’re both smart enough to know it.

Finally, there is yet another tier to consider: the moderators. Fox Business News’ panel has nowhere to go but up from the execrable CNBC debate two weeks ago, but they have to also show that they are willing to ask tough questions. I’d expect better organization of topics from the last debate, and more focus on topic, but don’t be surprised to see some challenges to Carson’s bio, Rubio’s credit card controversy, and Trump’s temperament. If Ted Cruz wades into that to crack the whip like he did the last time, he might end up being the big winner.