Live Updates Ahead of Tonight’s Democratic Debate

• The Democratic debate will begin at 8 p.m. Eastern in Detroit.

• The debate will be hosted once again by Dana Bash, Don Lemon and Jake Tapper.

• The ratings for Tuesday night’s debate were down sharply compared to those for the June Democratic debates.

How did the last debate change public opinion polls?

The rematch between Mr. Biden and Ms. Harris has been billed as the must-see pairing of tonight’s presidential debate. But while the June debate featured a wobbly performance by Mr. Biden, and a breakout one by Ms. Harris as she challenged his record on race, it is hard to find much evidence that their first matchup had a lasting effect on public opinion.

Just over a month later, Mr. Biden holds the support of 32 percent of Democratic voters, according to the latest RealClearPolitics average. It’s exactly what he held heading into the June debate. He fell to 26 percent in the week that followed that first debate, but he has recovered nearly all of the support he lost.

Ms. Harris’ breakthrough did not prove durable, either. She briefly surged to about 15 percent, good for a three-way tie with Senator Elizabeth Warren and Senator Bernie Sanders in second place. But Ms. Harris has slipped back to around 11 percent in the latest polls, about halfway in between her post-debate peak and the 7 percent she held heading into the contest.

It’s a reminder that the debates, despite their high viewership, don’t always have a lasting effect on the race. A wave of positive headlines can swing the polls, but aren’t guaranteed to bring about a lasting change in public opinion.

[Moderate candidates struggled in Tuesday’s debate. Will Mr. Biden make a stronger case tonight?]

Mr. Biden and Ms. Harris aren’t just competing for a new round of positive headlines in tonight’s debate. Over the long run, they are competing for the support of black voters, who are poised to make up around 20 percent of the national Democratic electorate.

It helps explain Ms. Harris’ attacks on Mr. Biden’s record on race, busing and segregation in the first contest, yet Mr. Biden nonetheless maintains a lead among black voters in recent polls, including a commanding 53 percent to 7 percent advantage over Ms. Harris in the latest Quinnipiac University poll.

Ratings fell sharply for the Tuesday night’s debate.

Blame summer doldrums, political fatigue, or competition from two hit reality shows. Whatever the reason, viewership for Tuesday’s Democratic debate on CNN fell sharply from the massive audience that tuned in last month for the opening round of debates in Miami.

About 8.7 million Americans watched the debate portion of Tuesday’s event in Detroit, according to Nielsen statistics released on Wednesday. The average audience was roughly 8.2 million for the entire two-hour-and-43-minute broadcast, including candidates’ opening and closing statements and a Hollywood-style video that CNN aired at the start.

By comparison, about 15.3 million people watched the first debate night in Miami, which aired on MSNBC, Telemundo, and NBC’s national affiliates.

Still, CNN’s audience was easily the largest on television for the night, edging out “America’s Got Talent” on NBC and the season finale of ABC’s “The Bachelorette.” The viewership was roughly the same as a Democratic debate that aired on CNN in January 2008, at the height of a tight primary between Hillary Clinton and Barack Obama.

The Nielsen numbers did not include viewers who watched online or via streaming video.

Reported and written by Nate Cohn and Michael M. Grynbaum.