While the inverse proportionality of political conservativism and Christian discipleship may not be completely true, I do think there is truth in it.
Several weeks ago I quoted a famous liberation theologian, saying essentially that Christian devotion to the Kingdom of God means opposition to economic poverty and social injustice. One conservative friend responded quite ironically that, yes, this is true... and that is why the thousand year reign of Christ after the rapture is so important. Of course, this response certainly does not reflect the point of liberation theology... especially in the context of the quote I was citing. My friend--innocently, I hope--missed the point. While liberation theology, at its best, calls us into participation with God's redemptive and liberating activity and reminds us of the fundamental importance of God's solidarity with the "least of these" (Matthew 25) in history, my friend had reserved Christian hope to heaven and to some future event outside of history and thus reduced the gospel to inward piety and personal belief (he made the opposite mistake that some liberation theologians make). Social justice, liberation, solidarity, equality, ministry (by a certain definition)--these are not his problem because, in his view, they're God's problem and God will take care of them after the rapture. When Christianity is reduced to getting into haven after death, hanging out with Jesus in the eschaton while the world burns, then there's no real rationale for anxiety over injustice or struggle for equality. Just wait for Jesus and read your bible in the morning... there's plenty of room for political structures which protect your wealth and preferentially benefit the powerful.
It wouldn't be fair to identify political conservativism, as such, with preference for power, indifference toward the poor (since it's their own damn fault anyway), American nationalism, militarism, and special interest in the accumulation and preservation of wealth... but, let's face it, those impulses are more common within conservative political ideologies. This is the hint of truth in my original, though faulty, proposition that political conservativism and Christian discipleship are inversely proportional. While not all conservatives are this way, those impulses listed above which does describe some conservatives are, in fact, inversely proportional to Christian discipleship. It is only when Christianity is reduced to a purely "spiritual" and ahistorical belief system--as opposed to a normative claim concerning not only inward piety but also the being and action of God in history and the corresponding human response--that one can possibly defend a position which gives preferential attention to the wealthy and, indifferent to the oppressed and marginalized, prefers to let social justice and injustice work themselves out. In other words, for a Christian to live with an ideology which perpetuates poverty and injustice, they have to have a Christianity that allows them to do so; a Christianity which, as such, can only be reductionistic. No Christianity which reserves concern for the poor and the marginalized to some future event and reduces discipleship to personal piety can ever truly be called authentic biblical Christianity.
So even tough it's not conservativism, per se, which is incompatible with and inversely proportional to authentic Christianity, there are impulses in political conservativism for which one must carefully watch out.