One of Barack Obama's major campaign themes is the promise that he will "unite" America. Obama is an incredibly skillfull campaigner, so I must assume that he wouldn't be pushing this trope unless there were good reason to believe that it works. Of course, Obama is far from the only politician to promise unity. Remember when George W. Bush promised that he would be a "uniter, not a divider"? That was a fairly successful campaign theme too.
This emphasis on unity for its own sake seems misplaced. After all, unity is really valuable only if we are united in doing the right thing. Being united in doing the wrong thing is surely worse than being divided, if only because division reduces the likelihood of the harmful policies being enacted. And even if the policies proposed by the would-be "uniter" really are beneficial, it's not clear why broad unity in support of them is preferable to just having enough votes to get them passed.
Ultimately, the ideal of unity is antithetical to democracy itself, which relies on constant competition and division between parties. When democracy works well, it is precisely because of our divisions, which check the power of incumbents and ensure their replacement by their opponents if the voters decide they have screwed up badly enough. If we really value unity for its own sake, perhaps dictatorship or one party oligarchy would be a better form of government.
Despite its vacuousness, unity rhetoric seems to be popular. Popular enough that both conservative and liberal politicians routinely resort to it. Popular enough that a brilliant candidate like Obama has made it a centerpiece of his campaign. Popular enough that nationalists, socialists, fascists, and communists have all made effective use of it. Remember "One People, One Fuehrer, One Reich"? No, I am not saying that Obama (or Bush) is like the Nazis and Communists. Far from it. However, the Nazi and communist examples do dramatically illustrate how unity doesn't have any intrinsic value. The achievement of national unity made these regimes even worse than they would have been otherwise, not better.
The interesting question is why people find the idea of "unity" so appealing. I tentatively conjecture that the popularity of unity themes is in part related to rational political ignorance and voters' lack of incentive to consider the issues carefully and systematically. Because voters don't have much incentive to consider policy issues in detail, they often fail to get beyond the warm, fuzzy feelings that appeals to unity inspire. If voters were more sophisticated in their thinking, they would not so easily yield to this temptation. They would at least ask: "What is it that he wants to unite us to do?" They would also recognize that tropes about unity are one of the most common excuses for flawed, inefficient, and oppressive policies. As economist Dan Klein explains in his article "The People's Romance," the unity theme has a long and sordid history.
UPDATE: Various commenters and others claim that Obama is merely expressing the fact that Americans share common goals. Even if this is true, it still doesn't prove that the unity trope is a good thing. After all, there is no value to having common goals if those goals are the wrong ones. Unity has value only in so far as it can be used to promote beneficial ends, and is positively harmful otherwise. Moreover, I doubt that Obama (and previous practitioners of unity rhetoric) merely seek to express a preexisting unity. After all, if the unity they appeal to already exists, there is no need to support their campaigns in order to promote it. Rather, Obama, Bush and many others promise to provide a greater level of unity than existed before. That is surely what Bush meant when he promised he would unite rather than divide, and what Obama seems to mean today.