The Main Event Well, duh. The two heavyweights. The only two who have previously run for president. Mr. Biden and Mr. Sanders represent the ideological poles of the primary field. This is the marquee matchup of the two-night debates. And both have plenty of tape to review. Mr. Sanders can watch how Mr. Biden navigated the multi-candidate 2008 debates. And Mr. Biden can study how Mr. Sanders attacked an establishment front-runner in his primary debates with Hillary Clinton.

The Next Generation Flanking Mr. Biden and Mr. Sanders will be two fresh faces of the Democratic Party’s future, each making the case that the future is now. Mr. Buttigieg has displayed an uncanny calm in national television appearances, and it can be easy to forget that the debate will be his introduction to millions of viewers. Ms. Harris’s best national moments have been at Senate committee hearings, where the former prosecutor interrogated hostile Trump administration witnesses. But she was in control of those settings. Debates are far less predictable.

The Historic Age Gap Mr. Buttigieg, 37, would be the youngest president in American history. Mr. Sanders, 77, would be the oldest. Separated by four decades — and standing one spot apart — both seek an important constituency: millennials. Younger voters have formed the bulwark of Mr. Sanders’s base for four years and could be a key for Mr. Buttigieg. Mr. Sanders has won over many of them with his revolutionary message. Mr. Buttigieg is hoping to connect as a millennial himself. Which message and messenger will appeal more strongly to the audience?

A Mayor Confronting Tensions Mr. Buttigieg may face questions about his handling of police and race relations in South Bend, Ind., both before and after a black man was fatally shot this month by a white officer who had not turned on his body camera. Mr. Buttigieg soared to the top tier of candidates mostly from support from white voters and donors but has made plain his need to earn more backing from black voters and other minority groups. His leadership in South Bend could also become an issue for liberals concerned about racial and social justice.

The Gentlemen and Women from Washington There will be five current and former senators lined in a row: Mr. Biden, Mr. Sanders, Ms. Harris, and Senators Kirsten Gillibrand of New York and Michael Bennet of Colorado. The Senate is a body built on collegiality and obscure procedural rules like cloture votes. With so many senators onstage together, will any of them get tripped up trying to explain — or objecting to a colleague’s explanation of — the kind of wonkish details that can make voters want to change the channel?

Gillibrand vs. the Leading Men If there is a candidate arriving at the debate hungry to break out from the pack, it is Ms. Gillibrand, whose campaign has not met early expectations. As the race’s most outspoken advocate for women, in a party where women fared strongly in the 2018 elections, she is hoping to rally female voters. If the debate among leading candidates like Mr. Biden, Mr. Buttigieg and Mr. Sanders devolves into an ugly round of bickering, can she break through with a memorable moment that impresses voters?

The Moderate Men in the Wings John Hickenlooper, Colorado's former governor, and Mr. Bennet, the state's current senator, have argued that socialism and Mr. Sanders’s agenda are unrealistic and unwise for the Democratic Party. Both are vying to be a moderate alternative to Mr. Biden and could take aim at Mr. Sanders, who has flashed a willingness to punch back. Neither has gained much traction, but each could provide an interesting subplot. (As could the fact that Mr. Bennet was once Mr. Hickenlooper’s chief of staff.)