Niebuhr on the Religious Left with some context

Elizabeth quotes a passage from Moral Man and Immoral Society:

Sentimentality is the peculiar vice of liberal Protestantism. By adjusting its faith to the spirit of modern culture it imbibed the evolutionary optimism and the romantic overestimates of human virtue, which characterized the though of the Enlightenment and of the Romantic Movement…

In spite of the disillusionment of the World War, the average liberal Protestant Christian is still convinced that the kingdom of God is gradually approaching, that the League of Nations is its partial fulfillment and the Kellogg Pact its covenant, that the wealthy will be persuaded by the church to dedicate their power and privilege to the common good and that they are doing so in increasing numbers, that the conversion of individuals is the only safe method of solving the social problem, and that such ethical weaknesses as religion still betrays are due to its theological obscurantism which will be sloughed off by the progress of the enlightenment.

The "liberal Protestantism" that Niebuhr is referring to isn't the same as the liberal evangelicalism that animates a large chunk of contemporary progressive Christianity and is only somewhat related to the theology of mainline Protestantism.  What Niebuhr is referring to is the Social Gospel of the early 20th century as articulated by Walter Rauschenbusch.  And Niebuhr's criticism is quite apt, Rauschenbusch was very much an idealist who believed in the inevitability of the Kingdom of God:

As long as organized sin is in the world, the Kingdom of God is characterized by conflict with evil.  But if there were no evil, or after evil has been overcome, the Kingdom of God will still be the end to which God is lifting the race.  It is realized not only by redemption, but also by the education of mankind and the revelation of his life within it.

Niebuhr's chief criticism (found in Children of Light and Children of Darkness) was that this idealism ignores the very concrete reality of original sin and the simple fact that humans are irrevocably flawed. 

Considering that evangelicals of all stripes have largely the same position on original sin (it exists and can be forgiven but not overcome), I think it's likely that the liberal evangelicals who are growing in prominence don't believe in an ideal state that can be achieved by human striving.  But instead hew closely to the belief that only Jesus can inaugurate the Kingdom of God. 

Niebuhr's criticism is well-taken, but in this case, I don't think it applies.