What would cause you to vote against your party or candidate?

A legitimately better option.

I won’t vote for a candidate who has been shown to actually be a crook or a hypocrite. But that does not mean I will vote for the opponent.

Consider this statemet from the Washington Post:

The most serious, practical, pressing problem with politics as tribal conflict is the inability to draw moral lines against offensive, unethical and dangerous behavior.

If our public life is reduced to the binary choice between one side that we think will save the country and another that will surely destroy it, then leaders from our side are always preferable, no matter what norms they violate or institutions they wreck. If the victory of our cultural team is all that matters, then the methods our leaders employ become secondary to the point of insignificance.

And objecting to objectionable methods is seen as giving aid and comfort to the enemy.
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If a candidate said they were against abortion that would be a deal breaker. If they wanted to weaken labor laws, such as Janus vs. AFSCME, which weakens unions, that would be a deal breaker. Those are just two.

Common sense.

Common sense must be applied at some point. I’ve never seen a candidate that agreed with me on everything nor one that disagreed on everything. You have to pick your battles but insure that even if you lose some, you still live to fight another day. Fundamental Change is a good catch phrase but it is typically irreversible and very dangerous. A candidate that disagrees with me AND promises ‘Fundamental Change’ would never get my vote regardless of what the opposition promises.