Final viewership numbers won't be available until Friday afternoon. But the so-called "overnight" ratings, recording how many households tuned in, showed a 15 to 20 percent increase over Wednesday, which was night one of the two-night event.
Wednesday night's debate on NBC, MSNBC and Telemundo had a 12.3 household rating, in the "overnights," and Thursday night had a 14.1 rating.
The total viewership on Wednesday night was around 15.3 million viewers, just shy of the all-time ratings record for Democratic party primary debates.
The record was set in October 2015, when 15.5 million viewers tuned in to CNN for the first primary season debate between Hillary Clinton and Bernie Sanders.
Given the strong household rating for Thursday night, the total viewership could be in the ballpark of 17 million. Nielsen will distribute the actual viewership numbers Friday afternoon.
The high ratings have exceeded the expectations of NBC executives and surprised others in the TV industry. Democratic officials have cheered the news.
"The more people that watch our candidates, the more people are gonna like our candidates," DNC chair Tom Perez told the Washington Post.
NBC clearly benefited from its decision to carry the debate across three channels, including in Spanish on Telemundo.
And the two-night event benefited from enormous early interest in the presidential election.
The ratings still fall far short of the Trump-fueled records for Republican primary debates that were set in 2015. Trump's first time on the debate stage, in August 2015, helped attract 24 million viewers to Fox News. His second time, the following month, helped draw 23 million viewers to CNN.
All of these totals come from Nielsen's television ratings system, counting viewers who are at home watching on TV sets.
The vast majority of big event viewing still happens that way, but online streaming is a growing factor as well. This week's debates were live-streamed on YouTube, Facebook, Twitter, and NBC's own web sites.