as a Christian, i believe it is a sin to vote against one’s own conscience.
i’m with madison:
“It is the duty of every man to render to the Creator such homage and such only as he believes to be acceptable to him. this duty is precedent, both in order of time and in degree of obligation, to the claims of the Civil Society.” (from Imprimus, Sept. 2012).
That said, I would fear any church that would bring sanctions against a member for their vote.