The tension/paradox is easily stated.
Rhetorically, the political theology many progressive Christians espouse is Anabaptist. The rhetoric is anti-empire. Jesus is Lord, Caesar is not.
But in practice, the political theology of many progressive Christians is Niebuhrian. That is, Christians must take and use the power of the state to address our social and international problems. The focus is upon electoral politics and democratic engagement: voting, calling Congress, etc. Jesus may be Lord, but in this unjust world Caesar is how we get stuff done. That's Niebuhrian realism.
In short, it seems to be that a lot of progressive Christians want to be Anabaptist and Niebuhrian at the same time. Or adjust our position when it suits us. Anti-empire when you need to denounce an administration. Niebuhrian when you need to win an election.
Why does this paradox exist?
I think it's because progressive Christians have an anemic ecclesiology. Progressive Christians aren't known for showing up on Sunday mornings.
Progressive Christians resonate with Anabaptist, anti-empire political theology as it aligns well with the language of the prophets--indictment of oppression and injustice--which connects with the social justice impulses of progressive Christians. But lacking a robust ecclesiology, church as counter-cultural polis, progressive Christians are forced to turn to the state as the only player able to address the oppression and injustices they are calling out. Without a church, democratic engagement--guided by Niebuhrian political theology--is the only tool available to make the kingdom come on earth as it is in heaven.
Thus the paradox running through much of progressive political theology.
Denouncing Caesar while embracing Caesar.