He's not so much liberal as apolitical. He's staunch on the 2nd ammendment and was the first to point out how soft McCainiac was on guns.
His sidekicks, Chuck and Bernie are rockribbed conservatives and write most of the gotcha questions he springs on the lib senators.
He is a cantankerous old fart but he brings opportunities to hear the Senators and lib broadcasters with their guard down. You'll hear comments pass the lips of Brokaw, Rather, LIEberman, etc that you won't hear anywhere else.
I doubt you would have heard Alter make his comments anywhere else near a microphone.
For what it's worth, he refuses to have on Jennings or Hillary! (Of course he promised never to have Gore on and went back on that.)
We watch him on MSNBC in the morning and prefer him over GMA, Today Show, etc.