Many Christians are unaware of the principle of co-belligerency in matters of public life. Briefly, it is based on two biblical principles: natural law (Rom. 2:14-5) and the commonweal (Jeremiah 19:7). This matters for this election and the Kingdom of God (Matthew 6:33).
1. One does not need to be a Christian to discern basic principles of right and wrong. In fact, some non-Christians understand morality better than some Christians. This has nothing to do with salvation, but with moral knowledge, in politics and elsewhere.
2. The church is not the whole of life. Christians are citizens of heaven first, but also citizens of earth and put here to have dominion and be salt and light. We should seek the good of the city to which we are exiled, as Jeremiah put it.
3. Therefore, it is no sin for a Christian to vote for someone who is not a Christian, if that non-Christian has more moral and political knowledge than his opponent, and if he or she is more likely to contribute to the commonweal. Mitt Romney would not be Pastor-in-Chief, but Commander-in-Chief. His beliefs align with those of our Founders far better than our present President, who, in his own words wants to "fundamentally transform America"--meaning: undo the Constitution and cut America down to size. (See "2016: Obama's America" on this for a keen abductive argument that explains Obama to a "T".) Romney has experience in fixing dire problems, such as the Olympics and failing businesses. Obama has fixed nothing, but has exacerbated economic, moral, and military problems nearly beyond control.
4. Therefore: Vote for Mitt Romney and Paul Ryan and contribute to their campaign any way you can.