Dad is probably what you'd call a "Reagan Democrat" - leans Dem, votes Dem most of the time, but tends to be fiscally conservative and leans left socially. He's perfectly open to voting for a Republican if he thinks a Democrat screwed up. He thinks both parties are equally corrupt and distrusts all politicians regardless of their stripes.
Dad had a few misgivings about Obama in 2008 pertaining to his readiness to lead, but McCain's selection of Sarah Palin as his running mate cemented his vote for Obama. Though he had a few lingering doubts about Obama, the idea of a President Palin horrified him. He voted for Obama, but the enthusiasm quickly turned sour.
He became severely critical of Obama's decisions regarding the military, funding, infrastructure, cabinet appointments, immigration, construction projects, energy, executive orders (especially the mini-DREAM Act executive order).... During this campaign he has been critical of Obama and defensive of Romney. On more than one occasion he'd suggested that it was time to let a "business guy" take a turn at leading the country. Trying to persuade him to vote for Obama again seemed a futile effort.
When it looked like all hope was lost, Mitt stepped in it. Bigtime.
Dad saw Romney's disgusting reaction to the Libya/Cairo attacks in contrast to Obama's dignified, statesmanlike approach and said unequivocally, "That's it, I'm voting for Obama."
I hold onto the hope that this one experience isn't just an isolated incident.