David Friedman offers some rational reasons to hope Obama wins, and also discusses the emotional aspect of such valuations, in particular the Us vs. Them attitude that pretty much everyone is affected by to some extent. I think he misses one important point however, that matters because in this election we have an incumbent running for reelection. Therefore this vote carries an extra significance, which is an expression of popular judgement on the past performance of a public officer, i.e. the President. A critical feature of democracy is that the public may express it's dissaproval of policy-as-fact rather than just it's approval of policy-as-a-promise. So, if Obama wins then basically the public has lost it's chance to give a concrete expression of it's dissaproval of the policies that both Obama and Romney advocate in any case. Therefore I would say that it isn't only the (minute) differences between the candidates that matter, but also those policies that both candidates support but it is the incumbent that has put into practice. In this respect even if Romney may be slightly worse than Obama one should still hope that Obama loses, even if he loses to Romney. That will show both parties, especially the Democrats, what it is that we don't like, instead of giving them carte blanche to do whatever they want.
Over at the Beacon Anthony Gregory explains it pretty well too (emphasis mine):
Mitt “double Guantánamo” Romney has no apparent philosophical objection to the Bush-Obama police state, to militarized law enforcement, to a president with truly despotic authority. Neither does Obama. Romney could very well prove to be worse in practice, but at least he’d run the risk of people noticing. At least the debate over civil liberties would return. At least half the country would no longer see the arrest of sick marijuana patients, the mass deportation of poor migrants, the targeted summary execution of American citizens, and the torture of whistleblowers as unfortunate but necessary evils for which the president only deserves some blame. Instead, the blame would fall directly where it belongs: on the man sitting in the Oval Office.
I could never recommend supporting Romney, who I think would take this country further down the path of deficit spending, corporatism, fiscal insanity, militarism, and Big Brotherism. But I think anyone concerned about civil liberties in particular should refuse to support the continuation of the current regime. Progressives concerned about the future of their party should be especially cautious. The Democrats will permanently be the party of the kill list and indefinite detention if Obama wins this referendum.