Saturday, Austin Cline ran a piece titled There Will Be No Religious Left. I agree with him 100%, though the reasons that come to mind are somewhat different than his.
A good place to start is here. In short, the churches that are prospering are the fundamentalist ones, not the liberal ones. Trying to use small groups to build an opposition to big groups isn't exactly a winning strategy.
It's worth asking why liberal churches aren't doing so well. The common portrayal is that people want something stronger, but there's another side. The other side is that when you aren't convinced that believing is your ticket out of hell, it's easy to slip. If your church/temple/whatever only demands a couple of beliefs out of its members, rather than belief in everything in holy book X, then you can only give up a couple of beliefs before you find yourself with no religious beliefs at all. I am not surprised to see ReligiousTolerance.org saying "It is common for young adults to drift away from the faith group of their youth. Some never return. The large liberal and mainline Christian denominations seem to lose large numbers in this way."
So on one hand, there may be plenty of people leaving liberal churches for fundamentalism, but there are also people who leave churches of all kinds for general irreligion--though not self-conscious rejection of religion. As I noted a couple of weeks ago, 14% of Americans have no religion, 7% either don't believe in or don't know whether there's a god, and 1% are atheists or agnostics. That's 6% of Americans who won't call themselves what they are because they've been taught "atheist" is a dirty word, and probably another few percent who've given up religion but won't give up God because it's never been presented as a serious option.
Instead of trying to forge a religious left, here's what we need to do: convince people that it's OK not to believe in God, and tear down some irrational liberal creeds on religion, like "all religions are good" or, worse, "all religions are true." Say that people need to get up and oppose the idea that oppose the idea that some religious texts are infallible. Get very involved in opposing fundamentalists on issues that affect everybody, like gay marriage and evolution. On evolution, drop the idea that there are separate spheres of religion and science (except maybe when arguing court cases) and just say the anti-evolution arguments are bogus. A few simple measures like these could easily build a block consisting of about 10% of America.