Americans who watched the political conventions on television opted for news networks with partisan fan bases to a degree unseen in recent years, another sign of an increasingly divided electorate as the nation hurtles toward the November election.
Fox News, whose prime time is a destination for conservatives, accounted for close to half — 45 percent — of the viewership of the Republican National Convention this week across the six major news networks, Nielsen said on Friday. In 2016, that figure was about 30 percent; in 2012, 36 percent.
MSNBC, whose prime time is popular with liberals, accounted for about 30 percent of Democratic National Convention viewership last week across the six networks — which also include ABC, CBS, CNN and NBC — up from roughly 18 percent in 2016 and 2012.
During the Republican convention, MSNBC lost about 70 percent of its average viewership from the Democratic conclave. Fox News’s average viewership more than tripled.
Television viewers’ turn to perceived safe spaces raises questions about the ability of political conventions — which reached a broader TV audience in the pre-internet era — to persuade undecided voters. And it underscores fears about a polarized information environment where Americans can receive little exposure to political ideas that run counter to their own.
“It speaks to the larger point that we are siloed in our media choices,” David Axelrod, the Democratic strategist and CNN political analyst, said in an interview. “We’re a polarized country, and that is reflected in the media choices we make. We have the opportunity to create virtual reality worlds that affirm our points of view.”
A nightly average of 21.6 million people watched the Democratic convention on live TV, compared with 19.4 million for the Republicans. The total television audience for both conventions fell roughly 25 percent from 2016, a sign of Americans’ increasing reliance on online outlets and streaming services to follow live events.
President Trump’s 70-minute acceptance speech on Thursday was seen by about 23.8 million live viewers, falling short of Joseph R. Biden’s remarks last week, which reached 24.6 million — a comparison likely to irritate the ratings-conscious president.
Neither candidate attracted the number of viewers who tuned in four years ago for Mr. Trump’s acceptance speech (32.2 million) or Hillary Clinton’s (29.8 million).
Because Nielsen excludes streaming views — which are difficult to credibly capture — its ratings reflect the habits of an older slice of the population that still watches traditional TV. Some political analysts argue that Nielsen ratings are an irrelevant indicator, given the role of social media and other online platforms in the country’s media ecosystem.
Still, Americans’ TV habits over the past two weeks offer a glimpse of a cross-section of likely voters.
Fox News’s dominance during the Republican convention was striking. Its audience on Thursday, for Mr. Trump’s climactic speech, was nearly 9.2 million, close to a prime-time record for the network. That was more viewers than watched ABC, CBS, CNN and NBC combined.
Though the channel’s most popular hosts support Mr. Trump, Fox News viewers heard some critical commentary about the president on Thursday.
The anchor Chris Wallace called Mr. Trump’s speech “surprisingly flat” and “far too long.” Brit Hume, an analyst, said the president “seemed to miss the excitement that he generates in himself when he’s ad-libbing.” Even the pro-Trump host Laura Ingraham, after calling the president’s speech “incredible” and “electric,” conceded to viewers, “Some are saying it was a little too long.”
On MSNBC, three Trump critics — Rachel Maddow, Joy Reid and Nicolle Wallace — lambasted the president’s address and interrupted the convention for several fact-checking segments. The channel’s ratings for the Republican convention were among its lowest prime-time weeks of the year.
For the Democratic convention, the picture was sharply reversed.
MSNBC clocked its highest-rated prime-time week in the network’s 24-year history, with a 10 p.m. average of 5.7 million viewers. Fox News’s viewership fell far below its usual prime-time average.
“What we saw in the last presidential election was that Clinton supporters distributed their attention much more evenly among a broader range of outlets, and Trump supporters concentrated much more heavily on Fox News,” said Yochai Benkler, a co-director of the Berkman Klein Center for Internet & Society at Harvard Law School.
“The fact you have such a high proportion of viewers of the Democratic convention on MSNBC does suggest, to some extent, a gravitation on the Democratic side toward a more partisan, viewpoint-reinforcing network,” Mr. Benkler said.
For Mr. Axelrod, an architect of Barack Obama’s presidential campaigns who helped oversee Democratic conventions in 2008 and 2012, the Nielsen trends speak to a wider development in the years since.
“We are more polarized than we were in 2012 and 2008,” he said. “The elasticity in the electorate is even less. It wasn’t great then; it’s even less now.”
Mr. Benkler wondered how many truly undecided voters had tuned into the conventions in the first place.
“It’s just a very, very small slice of the American public who have not yet made up its mind to go for Trump or not,” he said. “They aren’t going to be the news junkies that spend their time on 24-hour cable news channels.”
He said he was surprised to hear Fox News’s proportion of network viewership of Mr. Trump’s convention.
“Forty-five percent?” Mr. Benkler said. “I would have thought it would be even higher.”