Ezra Klein points to an interesting study at Vox. The study provides evidence that partisanship is driven more by fear and hatred of the opposition than by supporting the policies and politicians of your own party.
Basically, Republicans might not have liked Romney but they loathed the idea of a second Obama administration much, much more. The left wing of the Democratic Party might swoon for Bernie Sanders and Elizabeth Warren but most of them will eventually fall in line for Hillary Clinton because they (including me) think that the GOP has dived off the deep end.
I largely agree with Klein and find this deeply depressing, even though I am a small bit of proof in the study. I love being a Democratic Party member and think Hillary Clinton will be a fine President if elected (if not always as liberal as I would like). But I’ve got to agree that I find it impossible to imagine supporting a GOP candidate. There is not one GOP politician for whom I can say “I disagree with many of their positions, but I think they will be a fine leader of the nation.”