The second night of NBC News' Democratic debate -- headlined by Joe Biden, Bernie Sanders, Pete Buttigieg and Kamala Harris -- topped Night 1 by 18%. The first night attracted 15.3 million viewers. Thursday's debate was the most-watched in Democratic party history. Yet both of this week's debates didn't come near the 24 million posted by President Trump's first primary debate, on Fox News in August 2015. ALSO: Stephen Colbert beats Jimmy Fallon in total viewers in live post-debate shows, but Fallon is tops with demo viewers.