Taking up my own challenge, I've been thinking a bit about how one might construct a theological basis for political liberalism. Obviously this won't be of much interest to those who see no need for such a basis. And I should say that I am not quite sure I would claim this as my own view. It is just some thoughts in process.
1. The departure point for all Christian thinking about politics is 'Jesus is Lord'. This is both a factual statement - he simply does rule over all things, whether anyone acknowledges it or not - and a polemical statement - he, and only he, truly rules.
2. As both a factual and a polemical statement, 'Jesus is Lord' is a positive statement with a negative implication. The positive statement is to do with Christ: he really reigns and rules. The negative statement is to do with everyone and everything else: they do not reign and rule (in the same sense). So 'Jesus is Lord' is both a description, advanced in the face of opposition, of Christ's sovereignty, and a delimitation of the spheres of all other powers.
3. If Jesus is Lord, Caesar is not. Caesar can be and must be honoured and obeyed, but this is for the Lord's sake, not Caesar's. We should be clear here that 'Caesar' for us today includes both demos and despotes, the people and the politicians. Caesar does not have ultimate sovereignty. The 'will of the people' (whatever that means) cannot convey or carry such sovereignty.
4. In particular, Caesar is not Lord over the minds and consciences of people. Since Jesus is Lord over all, and over each individual, nobody else is Lord. This does not mean overthrowing all order - Caesar is still to be obeyed in his sphere, family order is still to be followed, in the workplace there will always be hierarchy - but these things are dramatically relativised. As soon as they step over into areas of conscience, they transgress.
5. Since Jesus really rules, any attempt by human beings to put his rule into practice through the political sphere is tantamount to blasphemy. It must include somewhere within it the idea that he does not currently rule, that he is not sovereign, and that he needs us to establish his rule for him. But he does really rule; it is unnecessary and even sinful to try to bring in his rule through political power.
6. Since the Jesus who is Lord is the Creator, there is a common good which can be served by politicians. Even amongst people with wildly differing views of what 'the good' is, there will be sufficient common humanity (which exists after the image of Christ...) for leaders to be able to identify and strive towards some goals which are genuinely for the good of all, and for which good arguments can be made. So within the limited sphere allowed to human leadership, there is positive good to be done.
7. Since the Jesus who is Lord is a Servant, Christian leaders should seek to serve - and in particular to serve those who oppose them. Therefore Christians should be at the forefront of developments for the good and freedom of those with whom they disagree.
Possibly more to come...