This week’s Democratic National Convention attracted an average nightly television audience of 21.6 million viewers, down roughly 18 percent from 2016 but still a respectable number given how many Americans have turned away from traditional TV sets in favor of online video and streams.

An examination of TV viewing patterns reveals a nation that remains deeply divided, politically and culturally. And it raises questions about which voters chose to focus on the convention and which voters tuned it out.

MSNBC, home of liberal favorites like Rachel Maddow and Nicolle Wallace, had the highest-rated prime-time week in its 24-year history. The channel’s mostly unfiltered coverage of the four-day Democratic jamboree easily ranked ahead of every other TV network on every night.

Fox News, the cable home of Trump cheerleaders like Sean Hannity and Laura Ingraham, devoted its 10 p.m. hour to convention coverage. The network recorded its lowest average viewership in the time slot since the start of the year.

Over all, live TV viewership fell 17.6 percent from 2016, according to the ratings agency Nielsen. Thursday’s broadcast, when former Vice President Joseph R. Biden Jr. accepted the nomination in Wilmington, Del., drew 24.6 million viewers, the week’s biggest television crowd.

Still, the ratings picture underscored concerns about a choose-your-own-news dynamic that is increasingly prevalent around the country, as Americans head into a high-stakes election reliant on information sources that can affirm pre-existing points of view.

The divide was not exactly unexpected. Fox News, the No. 1-rated cable news channel — and, this summer, the top-rated network in all of prime-time television — typically lags behind its rivals during the week of a Democratic convention.

Even so, Fox News drew a bigger audience on Thursday than CBS or NBC, including among younger viewers, a notable win over two broadcast networks that are available in more American households. Fox News’s coverage outranked CBS every night of the convention.

Few viewers — or Democratic officials, for that matter — knew quite what to expect.

With the coronavirus preventing the usual TV tropes of patriotic crowds and airdropped balloons, Democrats put on an entirely virtual show, mixing taped video feeds with celebrities on a Los Angeles soundstage and Mr. Biden speaking to a mostly deserted exhibit hall in a Delaware events center.

Glitches were few, to the great relief of the Democrats’ production team, which oversaw the event from a pair of control rooms in Wilmington and Milwaukee, the original site of the convention before its in-person elements were canceled. The convention had an interactive element, as well, with at least one viewer: President Trump, who reacted in real time, often angrily, to the events unfolding on his screen.

On Thursday, Mr. Trump opted to have his own say on the matter, phoning into Fox News for a live interview on “Hannity” — a breach of past protocol. Presidents have typically deferred to their electoral rivals on the week of a convention.

About 4.6 million people watched Mr. Trump’s half-hour appearance. After he hung up, and the network’s convention coverage began, the Fox News audience fell 36 percent.

Nielsen figures do not capture the entire universe of this week’s viewers, including those who watched on internet livestreams, a number that is difficult to credibly calculate.

As they did during the conventions in 2016, cable channels beat the Big Three broadcast networks. CNN won the week among viewers ages 25 to 54, the most important demographic in the TV news industry, beating even MSNBC. ABC had the highest ratings of the Big Three broadcasters.

The broadcast networks also suffered from weaker lead-ins than in past convention years: Live sports have been curtailed because of the coronavirus, and many TV productions are halted, leaving viewers with reruns and other less appealing fare.