The Christian has an unusual position in relation to the political world.
Fundamentally the Bible teaches that God controls who is in power (Romans 13:1) which means that in reality none of us can actually control who our leaders are: however we as individuals might vote, the candidate whom God needs to be in power will win.
‘The Most High rules the kingdom of men and gives it to whom he will and sets over it the lowliest of men.’
As an aside this doesn’t mean that all world leaders are godly, upright people. It is clear that often the exact opposite is true. Rather the leaders are in place who will bring God’s plan into fruition (e.g. Pharaoh, Romans 9:17).
Alongside this is the problem that Christians have a very clear blueprint for life; in three words this is the example of Jesus. No politician, party or manifesto can ever come close to this pattern so voting for them would always involve a compromise on principles.
Finally, sometimes the call to follow Jesus may create a conflict with the call to serve the country. In these situations, such as taking up arms, the requirement to follow Jesus overrides any other duty. With this in mind many Christians will choose to withdraw completely from the political/national arena so that the distinction is clear:
Dear friends, I urge you, as aliens and strangers in the world, to abstain from sinful desires, which war against your soul. Live such good lives among the pagans that, though they accuse you of doing wrong, they may see your good deeds and glorify God on the day he visits us. (1 Peter 2:11,12; NIV)