Neither.
They know that having Trump on the ticket means that they are going to have to answer questions like “How can you support that man???” – and they’re not able or willing to answer those questions.
And the republican quitters know that the Democrats are coming at them with all speed and fervor, and they don’t want to have to defend the shit that they have been supporting and voting for for the last couple of decades.
Finally, politics was fun when all the republicans had to do is say “Obama bad”. But now that they have responsibility for governing, it’s a lot like work. And they don’t like being responsible for anything.
Basically, they’re bailing because they have nothing positive to say about their accomplishments, and they’re fricking cowards about standing up for their beliefs.