The Hill reports viewership for last night's debate was ca. 10 million, quoting preliminary Nielsen numbers. That's down from the first debate just over a month ago.
A cynic would call last night "the competition for white Democrats' support." You might well think that, I couldn't possibly comment (classical reference).
The drop reflects, to some degree, the absence of aspirants "of color" on last night's stage. The absence was, we are told, an unintended consequence of CNN's supposedly random drawing process.
Depending on whether you include Tulsi Gabbard who Wikipedia says is ≤ 1/4 Samoan, there are either 4 or 5 "persons of color" among the 20 Democrats angling for the nomination. All of these debate tonight. If viewership jumps tonight, their presence could be a factor.
COTTonLINE predicted some years ago that, following the Hillary debacle, the Dems will never again nominate a white for president. Perhaps instead of "nominate" I should have written "elect," we'll know in under a year.