The average for all debates in the current campaign, however, remains well ahead of four years ago.

The sixth debate of the Democratic primary drew the smallest audience so far this election season.

Thursday night's telecast on PBS and CNN drew a combined 6.17 million viewers, trailing the 6.6 million who tuned in to MSNBC for the previous debate in November. The debate, featuring seven candidates for the Democratic nomination, also came a day after 16 million people watched the House of Representatives vote for two articles of impeachment against President Donald Trump.

CNN averaged 4.09 million viewers for its simulcast of the forum, with PBS drawing 2.02 million. The remaining 20,000 watched on CNN en Español.

The biggest TV audience for a primary debate in this election cycle remains the 18.1 million who tuned in to NBC, MSNBC and Telemundo for the second night of the first debate on June 27. That telecast set a record for a Democratic primary debate.

The all-time biggest audience for a primary debate for either major party remains 24 million for Fox News' first Republican debate in August 2015.

Despite Thursday's low, viewership for the primary debates in the current cycle remains well ahead of number four years ago for the Democrats. The eight telecasts (including two nights each for the first two debates) are averaging 11.71 million viewers, a 47 percent increase over the nine primary debates in 2015 and 2016 (7.97 million).