Trump excites the “base” but not independents or traditional Republicans who believe in free markets etc. The temptation for Congressional Republicans up for election is to use “strategic ambiguity” and have their cake and eat it. That is, say you support the Republican Presidential nominee but not embrace his positions, e.g like Ayotte and McCain. This way, you hope ticket-splitters vote for you to check and balance Hillary.
Unfortunately, Obama moves second. He will say Trump is unfit to be President, is not a Republican and will tar supporters with the same brush. This way he will seek to slice off the base vote from the non-base. A vote for a supporter is a vote for Trump. How will they check and balance a demagogue when they are not splitting with him now? Also – and somewhat unexpectedly – Trump is helping Obama out here by refusing to endorse Congressional Republicans employing strategic ambiguity. He refuses to endorse Ayotte and McCain because of criticisms etc. Hence,he is signaling to his base not to support them (not sure if his strategy makes sense but I will take him at face value).
So, on the one hand, Obama will attack anyone on the fence by saying they support Trump (hoping to peel off independents, “real” Republicans and ticket-splitters) and Trump will attack anyone on the fence by saying they do not support him (hoping his base will not support them?!!).
So, strategic ambiguity is going to backfire so you have to pick a side. Which side? Can you win if you support him and he loses your state? If the answer is a likely Yes, you support Trump (e.g. Rubio) and if it is a No, you Dump Trump. The more likely your state is to go for Hillary, the more plausible your “I will check and balance Hillary” argument and the less costly it is to Dump Trump. Hence, Toomey and Ayotte are likely in this category. If the state is 50-50 like AZ, your choice is difficult, eg McCain, but you have to pick a side otherwise both may not vote for you.