The Lesser of the Two Evils
Let's say there's a race for public office that you're interested in. And the ideal candidate is running for this office. But he really doesn't have a chance of winning because, when it comes right down to it, he's too much of a gentleman to do what needs to be done to win.
And let's say that there are two viable candidates for this office. And both of them have serious issues that makes you doubt whether either of them is suited to the job.
And let's further say that one of them is unquestionably much worse than the other.
So you've got A, who should get the job but can't; B, who shouldn't but who has a shot; and C, who really really really shouldn't but is the frontrunner. Whom do you vote for? A, because he's the right guy for the job? Or B, because your vote might keep C out of office?
Now suppose that you've got dirt on B. Do you publish?
The lesser of the two evils is, after all, still evil.