Nearly 16 million people watched the debut Democratic debate of the 2016 US presidential campaign, a party record but more than eight million fewer than the first Republican clash, Nielsen says.
The Las Vegas showdown between former secretary of state Hillary Clinton, her chief rival independent Senator Bernie Sanders, and three others surpassed the expectations of media experts and political analysts who had predicted viewership in the single-digit millions.
An average of 15.8 million watched the Tuesday night event hosted by cable giant CNN, according to television ratings firm Nielsen.
While that is record viewership for a Democratic primary debate, it was a far cry from the 24 million people who tuned in to Fox News on August 6 for the first Republican clash of the cycle, featuring the party's frontrunner Donald Trump and nine challengers.
That was by far the highest rating of any presidential primary debate in US history.
CNN hosted the second Republican debate, in mid-September, an event which attracted about 23 million viewers, the biggest-ever audience in the network's 30 years of broadcasting.
Few had expected blockbuster viewership for the Democrats, who featured Clinton and Sanders along with three others who are barely registering in the polls: former Maryland governor Martin O'Malley, former senator Marine veteran Jim Webb and Lincoln Chafee, an ex-governor of Rhode Island.
The ratings would undoubtedly have been higher if Vice President Joe Biden, who for months has weighed the prospects of a late entry into the race, had been on stage.