We'll never be able to change our nation's moral decline via politics. That is not the reason for politics, government, etc. The Gospel is the ONLY means by which people can be truly changed from the inside out.
That being said, I believe part of our mandate as believers is to -- as much as is possible -- be engaged in the culture as SALT and LIGHT (Matthew 5:13-16). I think in our nation, that clearly includes our involvement in civic affairs -- voting at the very least! (cf. Romans 13:1-8; I Timothy 2:1-4; Proverbs 29:2) To do less actually invites and encourages evil and persecution -- and it is a SIN.