Aides to Mr. Sanders rebuffed questions asked by my colleague Sydney Ember about whether he’d be able to stand so long on a stage, but they fully realize that his energy, vibrancy and ability will be under intense scrutiny from voters.

Warren takes incoming

Ms. Warren’s surge in the polls has finally reached the point where it has become problematic for Mr. Biden and Mr. Sanders. Both men have signaled that attacks could be coming, with Mr. Sanders drawing a pretty sharp contrast over the weekend.

It’s likely that some of those attacks will focus on her health care plan: how she would pay for it, whether she would raise taxes on the middle class, whether she would end private insurance plans and how she would accommodate union health plans.

Will Ms. Warren be ready with more polished answers than she has given in the past?

Whither Mayor Pete?

Pete Buttigieg, the mayor of South Bend, Ind., has the most field offices in the early states (tied with Ms. Warren) and a generous campaign bank account (only Mr. Sanders has raised more among Democrats). What he lacks is actual traction in the polls.

So far, most of his debate performances have been characterized by “West Wing”-style monologues. But there are signs he may pivot to a more aggressive tack Tuesday night in hopes of expanding his support. In recent weeks, he has nudged Ms. Warren and Mr. Sanders on health care and has been in something of a sniper fight with former Representative Beto O’Rourke of Texas.

There have been plenty of signs that the two young guns in the race may be headed for a shootout at the Otterbein University Corral. The question is whether that will be enough for either to make a mark.

Does anyone else break out of the pack?

Four candidates on the stage haven’t qualified for the next debate in November: the former housing secretary Julián Castro, Representative Tulsi Gabbard of Hawaii, Senator Amy Klobuchar of Minnesota and Mr. O’Rourke. Tuesday night is a now-or-never moment for them.