Daniel Schultz has a very good piece in Religion Dispatches on the main weakness of the so-called religious left. As Schultz puts it:

[Rev. Dr. William] Barber wants Christians to decide which side they’re on: that of the rich and powerful, or the poor, in whose corner God stands. This sort of appeal to the authority of scripture is well-used by both the religious left and right. It’s what Christians think they should do when sorting through options: Come, let us reason together by interpreting the word of God.

But as he further argues, this strategy doesn't work for two reasons. First, it requires "a shared understanding of the Christian responsibility to the poor." Second, because it also requires "a shared identity as Christians."

This understanding of Christianity is problematic, particularly for someone in the religious left. Contrary to the religious right, that is essentially a Christian movement with some conservative Jews in the mix, the religious left is a multi-faith coalition. Sure, the vast majority of those are Christian, but minority religions are better represented than in their right-wing counterpart. If the concern for the poor is primarily a Christian thing, then where does that leave their fellow travelers in the movement who don't happen to be Christian.

Their diversity makes for a shaky coalition, something that Mark Silk has argued before, because they are unwilling to use the tools of partisanship to achieve political victories. Luciano Gonzalez and I expand on why this phobia of partisanship is the major weakness of the religious left in episode 12 of The Benito Juárez Experience. In his Religion Dispatches piece Schultz further argues that

When liberals appeal to a shared religious identity to guide policy decisions, then, the argument becomes about the faith, not the policy. Jim Wallis has been talking about all the scripture passages concerning the poor for like 40 years, and it hasn’t changed much of anything, because conservatives don’t understand the priorities the same way.

He is right, right-wing religious leaders baptize policies as Christian after the fact, not the other way around. If you debate how a Christian policy would look like you could keep arguing forever with nothing to show as an outcome. Bragging about holding the high moral ground may be nice but doesn't change the desperate situation many Americans live in. Holding power and the ability to change that reality is much better, but to many on the left -particularly religious figures- power is something to fear rather than embrace.