You are a lifelong Republican and think Trump is not a conservative. You would never vote for him. You go into the voting both and see Clinton’s name and Trump’s name. What do you do? Either you bite the bullet and vote for Clinton or you abstain. Either way you have increased the probability that Hillary wins – OK not by much but since you are in the voting booth in the first place, you’re not a fully rational voter and so you care about the infinitesimal impact you have. So, you decide to make sure she’s hamstrung by a Republican Congress. You vote for the Republican Congressional candidates.
You would vote for Cruz but suspect he is a bit nuts. You vote for the Democratic Congressional candidates to make sure Cruz is ineffective.
(Kasich would actually be best all round but has no chance of making it.)