I've always used Micah 6: 8 as a good biblical foil in thinking through political issues:
He has showed you, O man, what is good. And what does the LORD require of you? To act justly and to love mercy and to walk humbly with your God.
Being horribly reductionistic justice has been the war cry of the right where as mercy was the preserve of the left ... and humility has no place in politics!?
With this in mind I think Trevor is on to something. When I was first becoming aware of politics (in the UK in the early 80s) Christians voted Conservative. As far as my parents were concerned sexual ethics and abortion were the deciding issues - justice trumps mercy, as it were.
But the times they were achanging. As I became a teenager, church youth groups were increasingly concerned with compassion to the poor and loving the stranger - mercy came to the fore. The right of centre Conservative party was considered to be individualistic and selfish. This gained momentum until finally the Tory party imploded and Tony Blair swept NuLabour to power in 1997. I was at Theological college in 1997. By then most Christians at college seemed to support Labour. Mercy trumps justice.
Are we seeing that now happen in the US? (Or am I just enjoying saying that Britain leads America in something?)
The western world is fed up with the Republican party at prayer. They are seen as being anti-gay, anti-women's rights and extremely hypocritical. All justice, no mercy and definitely no humility.
The message is clear - the political climate is changing. As usual that is both a good and a bad thing. No longer can Christianity be used as a pragmatic way to win votes. We cannot, no must not, appeal to the instinctive right wing agenda of justice unless we visibly demonstrate mercy at the same time, and all of this with a humble attitude. People will not listen to our position on abortion without seeing our care for pregnant teenagers. Our stance on homosexuality is easily dismissed without visible compassion shown to the gay community.
The bottom line is that votes are not cheap - the only way people will listen to us again is if we live the gospel we preach.