I disagree with your premise.
In my district, there will be at least three candidates for the congressional seat that is up for grabs. As a responsible citizen, I have the right to vote for which ever candidate I wish. This candidate, if elected, is responsible for representing the district where I live. This person is obligated to do what is best for us, to look out for OUR best interests.
If none of the candidates fit the bill, it would be unconscionable to vote for any one of them.
When it comes to integrity, principle, and conviction - Like Churchill said, "Never give in, never give in, never give in!!"
(nice use of the word "gadfly" in your post, it's a great word!)
Then please explain Churchill's support of Stalin and the Soviet Union on June 22, 1941.
In politics, many times one is required to make hard choices and support something or someone you despise as a means to a more important end.