THE NETWORKS VIEWERS TRUST (OR DON’T)…. For the second year in a row, Public Policy Polling put a “TV News Trust Poll” in the field, with some notable results.

PPP included PBS in the survey for the first time, and found the network easily surpassed its corporate rivals when it comes to public trust. The rest of the polled outlets — NBC, ABC, CBS, CNN, and Fox News — were relatively close with one another when it came to viewers’ perceptions of reliability.

But the shifts over the last year were interesting. NBC, CBS, and ABC each saw their trust levels increase over this point in 2010, by similar margins. CNN’s numbers were effectively the same.

Fox News, meanwhile, saw the biggest shift. A year ago, 49% of respondents said they trust the Republican network, while 37% did not. This year, both numbers took a turn for the worse — 42% now trust Fox News, while a 46% plurality does not.

There’s a simple explanation for this.

A year ago a plurality of Americans said they trusted Fox News. Now a plurality of them don’t. Conservatives haven’t moved all that much — 75% said they trusted it last year and 72% still do this time around. But moderates and liberals have both had a strong increase in their level of distrust for the network — a 12 point gain from 48% to 60% for moderates and a 16 point gain from 66% to 82% for liberals. Voters between left and center tend to be more trusting of the media across the board, which is why a fair number of them were still rating Fox favorably even a year ago at this time. But it looks like with a lot of those folks it has finally crossed the line to being too political to trust.

I have no idea what took them so long.

The PPP analysis concluded, “Democrats trust everything but Fox. Republicans don’t trust anything but Fox.” Sounds about right.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.