When should a partisan vote against party?

At what point do you vote against your preferred party?

I've been thinking a lot about this question, mostly because I hope several million Republicans will do so come November 6. 

So, in the spirit of intellectual honesty, I've thought about the times I haven't voted for the Democratic candidate. I've lived and voted in five states, so I've got a decent sample size. Here's what I've got:

- The Democrat is corrupt. In a House of Representatives election for a safe D seat, I voted against an incumbent that was under federal investigation for corruption.

- Voting even though there's not a plausible Democrat running for office. I will still vote in an election even when Democrats don't field a serious candidate. (I figure if someone cared enough to put her/his name on the ballot, I owe the courtesy of taking each office seriously.) One of my deal-breaker criteria is that whoever I cast my vote for should, at minimum, be able to handle the rigors of the office. The Basil Marceauxs of the world, sadly, don't quite make the cut. (He's a Republican, but plenty of nutters run on the D line, too. I just like this video.)

- The Democrat is an idiot. In one gubernatorial election, I had met both the Democratic and Republican candidates. I thought the Democratic candidate, while nominally a serious candidate, would've been a disaster if elected. In this case, the GOP candidate was a moderate and, more importantly, wasn't an idiot. Being governor matters more than most other offices in terms of being able to execute on basic, non-ideological governing functions. Therefore, I put a high priority on a candidate who is diligent, fair-minded, and competent. Ability to do the job matters more than ideology. 

Essentially, I'll vote for a Republican when there's clear corruption or incompetence. 

For Republicans who loathe Trump or just think he needs a more aggressive check on his power, what's your criteria for stepping away from the party line?