By Brian Stelter, CNN Business

(CNN) -- President Trump's Thursday night convention speech making the case for his reelection was lower-rated than his challenger Joe Biden's speech one week ago, according to overnight Nielsen ratings.

About 21.6 million viewers watched coverage of Trump's RNC address across nine cable and broadcast networks, down from 23.6 million viewers who watched Biden's DNC address on the same nine networks.

The totals may fluctuate slightly when final numbers are released later in the day, but Biden clearly edged out Trump.

The Democratic convention was also higher-rated than the Republican convention overall when the audience for all four days is tallied up.

Biden's campaign celebrated the overnight ratings win -- and embraced it as a potential way to get under Trump's skin.

The president's ratings obsession is well-documented. He has tweeted about ratings hundreds of times, often inaccurately. He raised the subject as recently as Friday morning, when he tweeted, "Great Ratings & Reviews Last Night. Thank you!"

He did not share any immediate reaction when the initial viewership figures showed him trailing Biden.

If history is any guide, he may tout Fox News Channel's ratings instead of the overall results. Fox News had by far the biggest audience of any channel on Thursday night, with upwards of 9 million viewers.

Fox News announced in a press release that it had the "highest-rated prime time average among total viewers during any convention in cable news history."

Nielsen's data sets count traditional TV viewership across America but largely leave out streaming and other digital forms of engagement, so the picture is incomplete.

And it is impossible to know how many people watched just Trump, just Biden, or both speeches.

So the ratings are not a proxy for voter behavior -- but they are notable for other reasons.

Average viewership for both 2020 conventions dropped from 2016, when Trump accepted the GOP nomination and Hillary Clinton ran against him.

That year, Clinton's speech averaged 29.8 million viewers and Trump's speech averaged 32.2 million viewers. (The 2016 totals cannot be compared exactly to the overnight ratings for this year, but they are directionally accurate.)

Biden's campaign relished the ratings victory on Friday: TJ Ducklo, national press secretary for the Biden campaign, tweeted to Biden rapid response director Andrew Bates and asked, tongue very firmly in cheek, "I always forget, does @realDonaldTrump care about his television ratings? Or is that not something he cares about? Like, do you think it'll trigger him that @JoeBiden's speech got way bigger ratings than his did?"

Bates replied even more sarcastically: "You mean the self-described 'Ratings Machine DJT,' who bet the farm on this & broke the law to exploit the White House as a prop while pretending he isn't in charge as the pandemic spins out of control and violence & division flare up? Nah, I think he'll be totally sanguine."

Later in the afternoon, Ducklo commented again: "Turns out Americans aren't really into the dark and dystopian view of the country that Donald Trump is presiding over as president."

Viewership of both of the Republican and Democratic conventions was rather weak on the broadcast side. Americans flocked to cable news networks instead. CNN noted in a Friday press release that, for the eight nights combined, CNN "topped all the broadcast networks -- NBC, ABC and CBS -- in total viewers and demos," meaning the different age demographics that advertisers target.

Overall viewership skewed older, as political coverage typically does.

Fox News ratings were relatively weak during the Democratic convention and incredibly strong during the GOP convention. Nearly half of all the people watching Trump's Thursday night speech on TV were watching on Fox.