This week’s two-day Democratic ­debate-fest is so sprawling that it looks more like a New England town meeting than a confrontation among presidential candidates. But don’t count on it being neighborly.

First, advice to everyone: Figure out who you are. This makes it easier to answer surprise questions and explains why both Sen. Elizabeth Warren (Mass.) and Pete Buttigieg have had such a good start. They don’t have to calculate on the fly. This helped Sen. Bernie Sanders (I-Vt.) in 2016, but it’s harder for him this year. He’s one choice among many, not the alternative to Hillary Clinton.

This event should not have mattered much to Joe Biden. Before last week, he could have contented himself with launching a couple of eloquent sallies against President Trump while good-naturedly parrying attacks from the rest of the field. There is a lot of goodwill toward Biden, so he might have won sympathy if his rivals had ganged up on him.

But Thursday night is now a big deal, thanks to Biden’s unforced error in hauling his relationships with onetime senators James Eastland and Herman Talmadge out of the segregationist past.

This was political malpractice. Biden’s lead in the polls is built on overwhelming support from African Americans, as my Brookings Institution colleague William Galston detailed. Yet it appeared more important to Biden to make his “I can work with everybody” point the way he felt like making it than to protect his greatest political asset: the trust and affection of black voters.

With some intense face-to-face campaigning in South Carolina and support from prominent African American political leaders, Biden sought to weather the storm over the weekend. He’ll have to work hard in the debate to reinforce his loyalists in the black community while showing all Democrats he has the discipline to go the distance.

If you’re in a lower tier, you have to decide on the one thing you really want voters to know about you or the issue you want to push to the fore. For some candidates, that’s relatively easy. Washington Gov. Jay Inslee will be talking climate change, while Julián Castro and Beto O’Rourke have been more upfront than anyone on immigration.

For others, it’s more complicated. Sen. Michael Bennet (Colo.) has a well-thought-through take on the flaws of our political system. His strength ­— that he transcends sound bites — is his weakness in this format. Sens. Amy Klobuchar (Minn.) and Kirsten Gillibrand (N.Y.) and Colorado Gov. John Hickenlooper must similarly move themselves into the consciousness of voters who have too many choices to think about.

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Sen. Kamala D. Harris (Calif.) is the year’s underperformer, while Buttigieg, mayor of South Bend, Ind., is the phenom. Harris had an awesome rollout, but she has languished. At times she runs left, at other moments toward the center. Who is she? To live up to her obvious potential, that’s the question she needs to answer. She took a step in the right direction with a forceful speech at the South Carolina Democratic convention on Saturday.

Buttigieg has been fluent and persuasive from the beginning, but the debate comes as he finds himself in the midst of a crisis at home after a fatal police shooting in South Bend. He spent Sunday afternoon not on the campaign trail but at a tense town meeting under challenge from African American constituents. Even before the shooting, his political imperative at the debate was to begin breaking out of his largely white and well-educated political base. Now, he will have to do so in the context of a genuine governing challenge.

Warren goes into the debates with the hot hand of rising poll numbers. She’s the only one of the top five appearing on Night One, which is not ideal. (Unfortunately for Sen. Cory Booker (N.J.), who got traction going after Biden’s problematic excursion into the olden days, he’s on Night One, too.)

Warren should try to set the tone for Night Two by talking about her plans to make the lives of non-elite Americans better and asking what the next evening’s crowd has to offer them. She should also be conversational and warm to show voters who admire her smarts but doubt her “electability” that she can prevail against Trump.

Sanders is losing ground in the polls, mostly, it appears, to Warren, and she won’t be onstage with him. One thing’s for sure: Implying as Sanders did on Twitter that Warren enjoys the backing of the party’s “corporate wing” won’t cut it, as Sanders himself later acknowledged.

The hardest part is that Sanders likes his old tricks but needs new ones. The issue isn’t his democratic socialism. It’s his gruffness that has worn thin and a feeling in large parts of the party (which he doesn’t actually belong to) that he did too little to help Clinton defeat Trump. Maybe he can draw headlines by announcing he’s becoming a Democrat. It would be a nice tribute to unity during a week when the incentives might favor brawling. Imagine the improbable trending hashtag: #BernieTheHealer.

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