Some will vote for the candidate who's vaguely on their side, but withhold volunteer work and/or money. Some will stay home or vote for third party candidates. A few (I don't think it will be more than that, but I've been wrong before) will vote for the other side. Well, actually, as an Obama supporter, I'm hoping for a lot of those moderate Republicans, but I don't know what to expect.
I'm vividly aware that this is a very important election, but this is mostly striking me as funny. It sounds like an Illuminatus game. And what's funnier is that all my life, I've heard complaints about Americans ignoring politics.... and now that we've got an election that people feel strongly about-- a lot of them are going to be less politically active as a result.