Ignoring for the moment whether or not he was right or wrong, the response to the concept was highly correlated with politics. Conservatives as a rule felt he had to be mistaken, and could come up with all sorts of compelling "scientific" reasons why he HAD to be mistaken.
He HAD to be wrong because because it suggested nuclear disarmament was essential for human survival, a position that threatened their concept of national defense.

Sagan's underlying logic was (again, not considering the actual correctness of his claims) went something like this:

1)A general, unrestricted nuclear war will create climatic conditions that will destroy everything that survives the hostilities. Nuclear combat is comparable to two men armed with Very pistols standing waist-deep in a room full of gasoline.

2)If that is the case, the use of nuclear weapons in a conflict is pointless, since there will be no winners, even if one side gains a clear military victory (i.e., it succeeds in absorbing much less damage than its opponent during the initial engagement). In the long run, everybody dies from the cold.

3)Consequently, nuclear weapons serve no military purpose, have no defensive role, and must therefore be immediately banned, even if unilaterally.

Personally, I do not agree with this. Nuclear warfare may be limited, not general, and nukes serve as a deterrent to attack or blackmail, even if they are never used, so the logic outlined above is not only faulty, it has been shown historically to be faulty. Note, this is not the same thing as saying that there aren't enormous risks in a nuclear-armed world. There are, but the reasoning of those three points is just flat wrong.

But the Conservative argument against Sagan was not based on on the validity of those points, instead it attacked his technical claims about albedo, soot, dust, fires, sunlight obscuration and the like, all motivated by their justifiable (in my opinion) fear of unilateral disarmanment, not by their knowledge of climate or the effects of weapons. Sagan's claims could not be verified or dismissed except empirically--by actually having a war and finding out, so criticism of his assumptions could not be easily dismissed either. They were terrified that Sagan's scientific reputation would lead to the adoption of his politics, so they attacked his science. The same thing happened with Rachel Carson and pesticides, Ralph Nader and auto safety, and with the debate over the carcinogenic effects of tobacco.

Something very similar is happening today vis-a-vis the global warming debate. If there is indeed global warming, then measures that Conservatives find unacceptable will have to be implemented: taxation, international concessions, regulation and bureaucratically imposed limits on extraction, production and consumption. In other words, their definition of freedom, unrestrained economic activity, would be threatened.

So regardless of whether or not the science on AGW is valid or not in reality, it MUST be wrong because its logical consequences are ideologically intolerable. It is easier, after all, to convince people the science is wrong than it is to explain why the measures are.

You're right, it is essentially a religious argument.

A) If evolution is right...

B) then Genesis must be wrong.

C) And if Genesis is wrong, the Bible is not infallible, literal, divinely inspired Truth.

D) And if that's the case, we have no proof of the existence of God or his requirements or plans for us.

Asserting A) is wrong keeps us well away from the slippery slope to D).

Keep in mind that whether Nuclear Winter, Climate Change, or Darwinian Evolution are scientifically valid is not the issue. For that matter, whether the position of the skeptics in each case is valid is equally irrelevant. The science must be attacked to distract from the real motivation for the resistance to it.

To be fair to our Conservative friends, they also accuse Libs of the same bias, that we reject the science on race, education, economics, politics, national defense, in fact, everything we differ on. Sometimes, they might even be right, but I won't dwell on that, since there is no way to conclusively decide those questions.

There's no Mystery to it. Our view of reality not only determines how we analyze data, it even influences how we perceive it.