I don't know much about the global winter theory beyond remembering when it hit the news. At the time, I thought it basically said that if you trash the atmosphere with enough nukes you will totally upset the climate which seemed reasonable enough if you don't try to get into details like how many nukes would it take and how long would it last. I don't think I really viewed it as science as much as a"it could happen" story. It did seem kind of tailored to fit the times and scare folks into pacifism.
Sagan was a really smart guy with a talent for explaining science in an approachable enthusiastic manner. As I recall he did have some fairly strong liberal beliefs that came out of the background once he got more famous. It is hard to not end up being political once you have some success reaching people as all the activists are always looking for someone to carry the banner for them.
There is a saying biology that fits: "Successful systems attract parasites" kind of fits our political process as well doesn't it.
joe