Since someone mentioned that they couldn’t find it in comments, and since it’s obviously relevant, mutatis mutandis to arguments below, it is probably no harm to link again prominently to dsquared’s classic post about how you actually aren’t obliged to vote Democrat, strong lesser-of-two-evils arguments are intellectually incoherent etc. Nut graf:
The argument I want to establish here is that the decision about whether or not to vote Demcrat (versus the alternative of abstaining or voting for a minor party) is a serious one, which is up to the conscience of the individual voter to make, and which deserves respect from other people whether they agree with it or not. Obviously in making that argument, I’m going to have to venture into a number of unpalatable home truths about the Democrats as they are currently organised (abstract: ineffectual, cowardly, surprisingly warlike, soft-right, generally an obstacle to the development of social democratic politics), but let’s get this clear right up front - voting Democrat might often be the right thing to do in any given case, depending on local conditions; it might even usually be the right thing to do. What I’m not going to accept, however, is that it is always or definitionally the right thing to do.