ST. LOUIS — The second presidential debate is shaping up as potentially Donald Trump’s final opportunity to keep his campaign from collapsing a month before Election Day.

With prominent Republicans already abandoning him in droves, other Republicans, including leaders in Congress, are looking to the debate to see whether Trump shows enough contrition to stand by him despite his vulgar comments about women. There’s been uncertainty about whether the Republican Party might shift resources away from his campaign in the final weeks or even try to replace him on the ticket.

Another layer of unpredictability is the debate format. The candidates are taking questions from undecided voters and will be onstage with stools rather than traditional podiums.