Given that President Trump positioned himself as an opponent of liberal culture more broadly than Democratic politics, the political victories celebrated by him and his allies are often unusual ones.

On Monday morning, for example, Trump’s son Eric tweeted this bit of victory-lapping.

The younger Trump appears regularly on Fox News, so he certainly has a personal reason to offer his congratulations to the network. But his broader point is clear: Fox News is being rewarded for its coverage — coverage that’s broadly favorable to his father — and CNN is being punished. CNN, of course, has been one of Trump’s favorite targets for what he argues is unfair coverage of his administration. And thus Eric Trump’s tweet: We, Trump allies and advocates, are winning. You, his opponents, are losing.

Fox News has consistently been one of the most-watched news networks in the country, and there’s one significant reason for that. Consider this chart from a Pew Research report released in January 2017.

In the campaign, Fox News was the most-watched network — because Trump voters watched it far more than any other network. Among Hillary Clinton voters, viewership was more diverse.

To oversimplify: If half of half the country watches Network A and the other half of the country watches Networks C through E, the most popular network in the country is going to be Network A.

Suffolk University regularly includes a question on trusted media sources in its polls. In the most recent national survey, conducted with USA Today, it found something similar. Democrats and independents trust several news sources; Republicans mostly watch Fox. (We’ve included MSNBC here because it’s on Eric Trump’s chart, but the sample size is small so there’s a high margin of error on data associated with it.)

That there’s overlap between partisanship and what’s watched is clear from the shows on Trump’s list (which, we hasten to note, is a snapshot of February 2018). The most-watched is Sean Hannity’s show on Fox News; the highest-ranked MSNBC show is Rachel Maddow’s. Hannity, here, is something like Trump in the Republican primary. Up against a lot of competitors, he finds that his core base of support that enjoys his extreme presentation and rhetoric is enough to move him into first place.

What’s interesting is how Suffolk’s data has changed since Trump first got into the race. Here’s the chart above, showing results from July 2015.

It didn’t break out PBS and NPR (meaning that those numbers are included in “other”). But notice the shifts on CNN: Democrats trust it more in 2018 and Republicans much less. But CNN is still trusted by half as many Democrats as Fox is trusted by Republicans.

We can parse Suffolk’s data to figure out the general composition of the three networks on Trump’s chart.

Now, about three-quarters of CNN and Fox News audiences are Democrats (blue) or Republicans (red), respectively. (MSNBC’s is strongly Democratic, but the same margin-of-error warning as above applies.)

In 2015, only about half of CNN’s audience was Democratic and only about two-thirds of Fox’s was Republican.

It seems, based on Suffolk’s data, that CNN has taken a beating from Republicans since 2015. Not all of the viewers CNN lost went to Fox, but that shift would help explain CNN’s decline.

Put another way, the ratings shift is another form of polarization. That the top non-Fox shows on Eric Trump’s tweeted list are political talk shows on less-Trump-friendly MSNBC is not an accident, certainly. Suffolk’s data suggests that Democrats are much more of MSNBC’s audience now than they were in 2015 (though, once again, margins of error!). If you are tuning in to cable news and are a fervent partisan, it’s unsurprising that you’d tune into Hannity or Maddow. It has nothing to do with the honesty of their reporting and a lot to do with the worldview they’re presenting.

Continuing that line of thinking, then, CNN’s problem isn’t just that it’s been targeted by Trump. It’s that its prime-time programs don’t present an unfailing worldview that aligns with partisan belief.

In 2018, that can be a disadvantage in the cable news game.