Just because the slogan “Don’t hope for a better life; vote for one” was used by the UK Tories in the late 1970s doesn’t, to me, mean that the McCain campaign “plagiarized” anyone by using it. The idea of plagiarism is that you have one writer taking credit for the work of another writer which we think is wrong under a variety of circumstances. But we don’t think it’s wrong in the context of political campaigning.
Barack Obama didn’t single-handedly write “Obama’s convention speech” or “Obama’s race speech” or “Obama’s competitiveness speech.” One gets the sense that Obama, who really did write a legitimately good book without recourse to a ghostwriter may play a larger role in his own speechmaking than is typical for a presidential candidate, but even if he doesn’t he’s not “plagiarizing” his speechwriters, he’s giving speeches. Given that context, I think the general principle is that when it comes to political sloganeering you’re free to borrow, modify, etc. as you like.