For one, the impeachment inquiry into President Trump is advancing. Voters will be eyeing how the candidates balance attacks on Mr. Trump with questions about Joe Biden’s role in the overseas business interests of his son Hunter.

For another, Mr. Trump’s decisions on Syria have thrown U.S. foreign policy a curveball. And interest will beis high in Bernie Sanders’s performance after his heart attack two weeks ago.

Here’s our live briefing, where you can also watch the debate live-streamed beginning at 8 p.m. Eastern. (Separately, enjoy our deconstruction of the architecture of memorable zingers.)