The setting could not be more apt. For Mr. Biden, Ms. Harris and Mr. Booker, who have most clearly pinned their primary hopes on earning the support of black voters, Detroit is emblematic of the case they’ve made for beating President Trump in a general election. It relies on exciting the coalition that previously supported Barack Obama, mainly young people and voters who are racial minorities.

This differs slightly from the arguments of some other Democrats, who tend to focus on winning back white working class voters who backed Mr. Trump in 2016, particularly in states critical to the Electoral College like Michigan, Wisconsin and Pennsylvania.

But before any of them can get to the general election, they have to stand out in the primary. Here’s what to watch for tonight.

Biden, Biden, Biden

This much is true: Mr. Biden remains the early front-runner in the Democratic primary, bolstered by overwhelming support from black voters. What’s unknown: If Mr. Biden has a repeat of his performance in the first debate, in which he was visibly put off by pointed attacks from Ms. Harris, can that support hold?