NavyLT started a thread today about the proposed bill that would relax D.C. gun laws. The bill was introduced and sponsored by both Republicans and Democrats in Congress. This bill will be favored by a large number of the members of one party, and might see opposition in large numbers from the other party. There will likely be members of each party on each side of the debate.
I would venture a guess that, in the past, at least one or two 'pro-gun' bills like this were solely introduced and sponsored by members of only one party, while opposed by the bulk of the other party. And I think it's safe to say which party landed where. AFAIK, Washington DC has been run, for the most part, by only one of the parties, and I believe that most all of the city-level politicians, who oppose this recent bill I mentioned, are from one particular party.
The Heller case was decided down, what many people believe to be, a political line that exists on the SCOTUS bench. The Chicago case will probably see a similar "party line" decision. It might also be safe to say into which political persuasion the 4-votes lean and which way the 5-votes lean.
A lot of times, you can make a safe assumption about about a politician's 'gun opinion' based on their party affiliation, and you'd be right, but not all time, especially nowadays. EddieNFL is right, it's not just a black & white, hard & fast rule. I think we've all come a long way over the past few decades.