Democratic presidential hopefuls US Vice President Joseph R. Biden (L), US Senator for Vermont Bernie Sanders and US Senator for California Kamala Harris (R) participate in the second Democratic primary debate of the 2020 presidential campaign season hosted by NBC News at the Adrienne Arsht Center for the Performing Arts in Miami, Florida, June 27, 2019. - US Senator for California Kamala Harris confronts Former US Vice President Joseph R. Biden about racism. (Photo by SAUL LOEB / AFP)        (Photo credit should read SAUL LOEB/AFP/Getty Images)
Democratic presidential hopefuls US Vice President Joseph R. Biden (L), US Senator for Vermont Bernie Sanders and US Senator for California Kamala Harris (R) participate in the second Democratic primary debate of the 2020 presidential campaign season hosted by NBC News at the Adrienne Arsht Center for the Performing Arts in Miami, Florida, June 27, 2019. - US Senator for California Kamala Harris confronts Former US Vice President Joseph R. Biden about racism. (Photo by SAUL LOEB / AFP) (Photo credit should read SAUL LOEB/AFP/Getty Images)
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(CNN Business) —  

Thursday night’s Democratic debate featuring Kamala Harris, Joe Biden, and eight other candidates was the highest-rated Democratic match-up in Nielsen ratings history.

About 18.1 million viewers tuned in for the debate across three TV channels — NBC, MSNBC and Telemundo — according to Nielsen (NLSN) data.

The previous record for a Democratic debate 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.

Part one of this week’s two-part event, designed to accommodate 20 candidates, averaged 15.3 million viewers on Wednesday night.

As political experts expected, Thursday’s debate sparked even more interest, likely because the highest-polling candidates were on stage together for the first time.

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 (FB), Twitter (TWTR), and NBC’s own web sites.