During political conventions, a blue channel and a red channel

Republicans watch this, Democrats tweet that.

Data from the last two weeks of rhetoric-filled political conventions show that Americans are increasingly funneling into separate partisan media silos. Right-leaning Fox News Channel scored huge numbers during prime-time coverage of the Republican National Convention. And Democrats, during their gathering that wrapped up Thursday, raced to liberal-skewing MSNBC and filled social-media platforms with millions of comments during speeches from President Obama and others.

“Fox did very good Republican numbers. During the Romney speech, they exploded,” said MSNBC President Phil Griffin, referring to the Aug. 30 acceptance speech by GOP nominee Mitt Romney. Fox News’ telecast during the 10 p.m. hour drew 9.1 million viewers, according to Nielsen. That was the highest tally for any single network during either convention, suggesting that the party faithful rallied around the network.

Overall, Fox averaged the highest 10 p.m. nightly totals for any network during both conventions (5.3 million vs. 5 million for runner-up NBC). But that victory was because of its stellar performance during the GOP confab.

During the three days of the Democratic convention, Fox was a distant third (2.8 million) among the cable news outlets behind CNN (4.5 million) and MSNBC (4.3 million).

MSNBC, on the other hand, grabbed the flag during the Democratic National Convention, beating coverage on ABC and CBS and posting growth of nearly 50% compared with the 2008 conventions, when Obama was first nominated. Over the last four years, the network has become a reliable meeting spot for left-leaning viewers who have championed Rachel Maddow and other hosts.

CNN and Fox News posted steep declines from the previous Democratic gathering — most likely because of deepening polarization as well as the fact that Obama was just being introduced to the American public four years ago but is now a familiar figure.

“Look how far we’ve come,” Griffin exulted. “Five, six years ago, there were discussions of whether to turn the lights off here and go home. We’ve really built our brand.”

Nielsen said 30.3 million tuned in to the final night of the GOP convention, while the last night for the Democrats drew 35.7 million.

The Democratic convention also drew far more commentary on social-media platforms such as Twitter, suggesting that young, tech-savvy users may be more receptive to the messages of Obama’s party.

On its blog the Crowdwire, digital measurement firm Bluefin Labs noted: “There were 1.3 million public comments about the 90-minute portion of the DNC [on Tuesday] with the widest TV coverage and the most important speakers, including San Antonio Mayor Julian Castro and First Lady Michelle Obama. The first night of last week’s GOP gathering, which included Ann Romney, pulled in 503,000 comments, or less than half” the Democratic total.

The partisan division in media usage isn’t viewed as a positive by some analysts.

“The conventions are infomercials,” said Brad Adgate, an analyst for Horizon Media in New York. “Years ago, when radio and TV were fledgling media, conventions were one of the events that put them on the map for Americans. Not anymore.”

And yet the conventions still retain the power to draw huge numbers — especially as at least portions of them are now covered live by 10 or more networks.

Nielsen estimated that a total of 25.1 million viewers watched Bill Clinton deliver an impassioned address Wednesday night on behalf of President Obama.

That was good enough to surpass the 23 million or so who watched the NFL regular-season kickoff game on NBC, which did not air convention coverage that night.

Whatever they say publicly, viewers don’t seem to mind the news media’s partisan divide, at least according to the numbers. Executives, however, tend to insist that the division is one-sided — and it’s all coming from the other side.

Fox News, not wanting to be branded a right-wing network, frequently points out that its election coverage features news anchors as opposed to pundits and its analysis includes commentary from liberals such as Juan Williams.

MSNBC, meanwhile, argues that Romney and other top GOP notables reliably appear on Fox News. MSNBC hasn’t even gotten an interview with Obama, Griffin says, although the president has sat down with Fox News, CNN and every broadcast network.

Viewing MSNBC as the liberal version of Fox News especially irks him. Griffin sees Fox News as ideological and MSNBC as more, well, fair and balanced. (Fox News declined to speak on the record.)

“When people say there’s some kind of equivalency there,” he said, “I go, ‘Really?’”

scott.collins@latimes.com