Reagan made a serious run for the Presidency in 1976 against Ford. As Governor Of California he stirred up immense dislike among liberals. There were an unending series of jokes about a President Reagan in those years. So a joke Impeach Reagan button would get a great reaction from half the audience, or more.
Certainly not. That would be a very obscure joke and wouldn’t be the least bit funny to 99% of the audience. Certainly the obvious one is what was intended. And that wouldn’t even make sense, since you wouldn’t impeach the President of SAG.
I haven’t looked at the state constitution, but I presume Reagan could have been impeached as governor of California. True he wasn’t governor at the time of the movie, but then Eisenhower and Johnson weren’t President at the time of the movie either. The buttons dated from earlier times.