...All right, there’s lots to unpack here. For one thing, based on national polls alone, Biden is still really in a tier by himself. It’s not just that polling at 28 or 30 percent is quite a bit higher than 17 or 18 percent. It’s also that the difference between Biden’s polling and the candidates below him falls within a range that, empirically, has been something of an inflection point as to who eventually wins the nomination or not. Candidates who are sitting in the mid-to-high teens — such as Warren and Sanders — don’t have a fantastic track record. A candidate in Biden’s position will still lose more often than not, but they have a considerably better record of success historically.

Of course, there’s no reason you should limit yourself to looking only at national polls. If you were building a predictive model at this stage, it would probably consist of some sort of amalgam of national polling adjusted for name recognition, early-state polling and endorsements, which are historically fairly predictive of nomination outcomes. In Iowa and New Hampshire, Biden looks weaker and Warren and Sanders (and to some extent Buttigieg) look a lot more viable. But endorsements are another story and those don’t look especially good for Warren and Sanders. Instead, Biden and Harris are well out in front in endorsements, although many potential endorsers are sitting on the sidelines.

The prediction markets deviate a lot from the objective data in the cases of Warren and Biden. They actually had her as being more likely to win the nomination than him (as of Sunday evening), even though he’s ahead in national polls and endorsements, and at worst tied with her in Iowa and New Hampshire (and way ahead in South Carolina). That isn’t necessarily wrong; it’s an early enough stage of the primary that I’d say there’s some room for subjectivity. But there are also some reasons to be cautious. The conventional wisdom has repeatedly expected Biden to implode when it hasn’t really happened yet. And frankly, the people trading in these markets — mostly younger and well-educated — aren’t your prototypical Biden voters.

And none of this makes it any easier to divide the candidates into tiers. For me, at least, the lines between the top several candidates are blurry. I’m pretty sure that I still like Biden’s chances better than Warren — as I said, that’s certainly where a statistical model would come out. But I wouldn’t wager a huge amount of money on that proposition. I think Warren has a few things going for her that Sanders doesn’t — less voter concern about her age, more room to make peace with the establishment and slightly better polling. But you could argue that they should basically be treated as tied.

I’m also not quite sure what to do with Harris. A “Party Decides” rubric that heavily emphasized endorsements and the ability to build a broad coalition would treat her as one of the favorites, while the polling wouldn’t. Then again, she’s had moments where she was polling better, and she could be poised to benefit if Biden falters among black voters or Warren does among college-educated ones. One reason to be pessimistic about the chances of candidates such as Cory Booker and Beto O’Rourke, in fact, is that if something did happen to one of the frontrunners, Harris would probably be first in line to benefit from that.

Overall, the best I can do is something like this:
Nate’s not-to-be-taken-too-seriously presidential tiers
For the Democratic nomination, as revised on Sept. 9, 2019

Tier
1. a. Biden

b. Warren

c. Sanders

d. Harris

2. Buttigieg

3. Yang, O’Rourke, Booker, Klobuchar, Castro

4. Everyone else

Note: Steve Bullock was demoted into the “everyone else” tier.
Even if you do have Biden, Warren and Sanders as your top three candidates (as I do), there’s no particular reason to draw a firm line at three candidates as opposed to some other number. If you’re just looking at national polls, then Biden’s still in a tier by himself. Prediction markets basically have it as a two-horse race between Warren and Biden. You can add Sanders to make it a top three… but factor in endorsements, and Harris probably also needs to join the group, which would leave us with four candidates. I don’t really put a lot of emphasis on money raised, as it hasn’t been a very predictive indicator historically, but if you did, you could add Buttigieg to the top tier and make it a top 5.
Perhaps this week’s debate will provide more clarity. If Warren has another strong debate and continues gaining in the polls, for instance, we might have a relatively clear two-way race between her and Biden. But the reality will probably be a lot messier.

Rival campaigns say she's gotten a free ride, but her allies warn that attacks will only backfire.

...with the Massachusetts senator now drawing massive crowds and surging in national polls, competing campaigns are starting to refocus on Warren, looking to blunt her momentum.

Aides to three rival candidates confirmed in interviews they’re revving up opposition research on Warren in preparation for the next debate on Sept. 12. Still others privately complained she’s gotten fawning treatment in the media as she unveiled a litany of ambitious plans without being pressed on where the money would come from to pay for them.

Warren’s supporters said the gripes amount to sour grapes from campaigns who didn’t take her seriously until she shot up in the polls, and are now watching with envy the organization she’s amassed while they were busy attacking each other.

Her opponents so far are treading carefully, wary of alienating Warren supporters whom they hope to win over eventually. Fellow liberal icon Bernie Sanders and Warren have long had a nonaggression pact, and various polls have shown overlap between backers of Kamala Harris and Warren. Biden, sitting atop the field, has been busy trying to deflect the brunt of attacks.

But surrogates for those top-tier candidates are starting to speak up.

The actress Susan Sarandon, while in Iowa supporting Sanders, threw shade at Warren last week. “[Sanders] is not someone who used to be Republican. He is not someone who used to take money, or still takes money, from Wall Street. He is the real deal,” she said.

An adviser to a competing campaign, who spoke on condition of anonymity, echoed some of that sentiment. If Warren attacks someone else’s record on the debate stage, the person said, she will get it back in return.

“Elizabeth Warren has gotten a pass in both debates,” Sellers said. He said he believes Warren has run the best campaign of the 2020 race, but added: “I think that at the end of the day, the biggest criticism of Elizabeth Warren is her inability to make her plans actual reality. There are a lot of voters — especially black voters — who will say, ‘A lot of this is pie in the sky and we want pie on the table.’”

Former Pennsylvania Gov. Ed Rendell, who is backing Biden, said Warren has avoided the kind of scrutiny that’s dogging Biden because she’s run a “mistake-free campaign” and has yet to be deemed a front-runner.

But Rendell said Warren won’t get a pass in the next presidential debate. He predicted, for one, that she’ll be pressed on how she would pay for Medicare for All.

“Elizabeth skillfully avoided answering the question: ‘Would your health care proposal cause taxes to rise?’” Rendell said. “She never answered the question … She’s done a great job avoiding it. She has to answer it now.”