What happened to the Religious Left?

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Amy Sullivan in Salon:

...[T]here was a time -- not so very long ago -- when the religious left was a powerful institution in American society and politics, when the term "religious" was not immediately assumed to connote "conservative."

Moral giants with names like Reinhold Niebuhr and Dorothy Day and Martin Luther King Jr. led intellectual and social justice movements.

It's nearly impossible to page through American history without coming across political causes that were driven either partly or entirely by progressive people of faith -- abolition, women's suffrage, labor reforms of the progressive era, civil rights, and any number of antiwar movements.

Just a few decades ago, venerable organizations like the National Council of Churches (NCC) made pronouncements that carried not only moral weight but political influence as well.

In short, the likes of Pat Robertson, James Dobson and Ralph Reed have not always dominated American politics; indeed, in the span of American history, the last three decades are an anomaly.

So, here's the thing: at a time when Republicans and the religious right are fused at the hip, Democrats and the religious left are hardly speaking to each other.

Why is this?