I certainly agree completely with John Sides about election mandates.
As John wrote with Andrew Gelman, "people often choose a preferred candidate and then rationalize issue positions to fit this preference." My favorite example of this was during the 2000 recount. As you may recall, Democrats were asking for a recount, conducted by hand, while Republicans wanted no such recount. Within days, partisans on both sides aligned themselves almost perfectly with their party positions on the general question of whether machines or people were better at counting votes. In fact, politically engaged students that I spoke with in November 2000 were convinced not only that machines (or people) were better at counting, but that they had always believed that machines (or people) were better at counting, and that their belief had nothing whatsoever to do with the current controversy.
One can imagine plenty of other complications. Consider a strong libertarian who votes against Republicans because of abortion, gay rights, and Bush-era civil liberties issues. Certainly, her vote was not for Obama's health care reform. People who read blogs like this one don't find that hard to imagine, but far more common are those who simply vote for Democrats (or Republicans) because they're always voted for Democrats, and really don't think about specific policy issues much at all. Parties and candidates do get something if they win elections: they get to hold office. But that doesn't mean that "the people" support everything those politicians want to do.
There are really good democratic reasons to contest things between, and not only during, elections.