Bob Costas was one of the most well-prepared, well-informed interviewers around during the years he did "Later".
His all-time favorite interviewee according to him, was Rush Limbaugh. In fact, he enjoyed having Rush on so much, that he asked Rush to stay on to tape additional footage that was then broadcast for a second night. Years later, Costas still said Rush was his all-time favorite interviewee. Limbaugh left quite an impression on him.
He was the protege of Hall of Fame broadcaster Jack Buck, who was a Republican and whom Costas has said was a great influence on his life.
I would be surprised if he is a liberal. He has mostly kept his political opinions and preferences to himself to avoid the taint of having to deal with the criticism. It is refreshing.
"Later" was one of the best shows of its kind, and Costas was one of the best interviewers of this generation. It's not his fault the show jumped the shark with Greg Kinnear.
I would not, as I obviously believe that he is a liberal. (Allbeit a slightly enlightened one.) That said, he does not broadcast his political leanings to the world like so many others, and for that I do give him credit.