People usually go for the candidate which engenders and/or embodies the most optimism.
http://www.ppc.sas.upenn.edu/cvabs.htm#ab119119. Abstract from Pessimistic ruminations predicts defeat of presidential candidates, 1900 to 1984.
Two psychological variables, pessimistic explanatory style and rumination about bad events, combine to predict depression and susceptibility to helplessness. We hypothesize that these variables should also predict the appeal of a presidential candidate’s message, and analyzed pessimistic rumination in Democratic and Republican nomination acceptance speeches from 1948 to 1984.
A blind, reliable content analyses showed that the candidate who was more a pessimistic ruminator lost 9 out of 10 times, and the victory margin was proportional to the difference between the candidates in pessimistic rumination. This was not due to a poor showing in the polls at nomination leading both to pessimistic rumination and defeat. Partialing out incumbency and standing in the polls around the time of nomination, the pessimistic rumination difference correlated with the victory margin (partial r=.89, p<.01). This basic finding was replicated for 1900 to 1944. The pessimistic ruminator lost 9 of 12 elections, and the difference in pessimistic rumination correlated with the size of the loss (r=.71). Three mechanisms are proposed by which pessimistic ruminators should lose: (a) voter aversion to depressive personalities, (b) the appeal to voters of hope, and (c) candidate passivity. As evidence for the third mechanism, pessimistic ruminators make fewer stops per day on the campaign trail.
These results suggest that the American voter, across historical periods, place a high premium on the appearance of hope.-----
Now I'm not saying Hillary Clinton is a pessimist. Just that Barack Obama is MORE of an optimist than Hillary, which bodes well for his future in the coming months.