It’s a line and a tactic that seeks to tap into some of Ms. Harris’s most memorable moments in the last two years: times when she has pressed Trump administration officials in Senate committee hearings. She has wielded her limited questions to maximum effect, most recently with Attorney General William Barr. But those settings are predictable. From the dais, Ms. Harris can run through a prepared list of questions and, ultimately, the witness is powerless to do much but answer them.

Debates are different, and the first one presents a big opportunity for Ms. Harris, whom many Democratic voters seem predisposed to want to like. Among her goals is to present herself as a credible opponent for Mr. Trump, someone ready, as she says, to go through his “rap sheet.” But a challenge facing Ms. Harris — and other Democrats at this early stage — is how to evince such toughness without doing bruising battle with fellow Democrats on stage.

How much will the candidates clash?

The candidates who debated on Wednesday night swiped at each other’s policy views occasionally, but there were only a few moments of fireworks. There were also no extended references to Mr. Biden or Mr. Sanders, even though many of the other candidates are seeking to cut into their bases of support.

Mr. Biden’s team in particular is bracing for a more aggressive approach from the candidates onstage on Thursday. Will they deliver that? And if there are sharp words, will they be directed only at Mr. Biden or Mr. Sanders, or will more rivalries crop up?

On Wednesday, for example, Tim Ryan and Tulsi Gabbard engaged in a pointed argument over the United States’ involvement in Afghanistan, and Julián Castro and Beto O’Rourke had an intense exchange over immigration. Ms. Warren — the poll-leader on Wednesday’s stage — stayed above the fray.

Can Buttigieg land the generational contrast argument?

Mr. Buttigieg, 37, has spent much of his campaign embracing his status as a millennial and arguing that the country should be represented by the next generation of leadership.

On Thursday, he will have the opportunity to draw the sharpest generational contrast yet as he stands next to Mr. Biden and near Mr. Sanders, two septuagenarians more than twice his age.