You don't think the question mark makes the point that it's an issue of contention in the article?
"You don't think the question mark makes the point that it's an issue of contention in the article?"
No, the question mark is not enough. As I said, when you would say it to someone, you could ask the question in a way to make it clear that you mean "Why should Christians bother to vote for Dems?".
But when you just read it as a stand alone, the question mark implies that you ask the question of why Christians SHOULD vote for Dems and you will give answers.
I really think it's better to make your major point right in the title, with the addition of the NOT.
In your e-mails, if you haven't sent them yet, I would even make it a statement: "Why Christians should not vote for Democrats". Remember, a lot of people really don't get much past the title, and this way you get the message to those too, then people can read the body of the message to understand your reasons.