Well, the history is a bit complicated.  But it turns out that the United States between the Appalachians and the Coastal Range, and south of the Potomac, has been something of a nature preserve for old kinds of Christianity that got crushed by events in Europe.  AmericanChristianity compares to western Europe in forms of Christianity but with eastern Europe in its insularity and slow/arrested development.

The long theological story is that Christianity has a succession of forms of growth in its first millenium: first the Christianity of Jesus and the Gospels consisting of units of small groups (around a dozen people), then a Christianity assigned to Paul and his writings consisting of cultic groups of hundreds.  Then there is a Christianity after Paul that attempts to monopolize all of society, indeed the world, and all of thought and human efforts into one all encompassing Church.

That last Big Christianity begins to fail with the Catholic/Orthodox schism and the breakdown takes nearly a millenium itself.  With it the authority of all writings outside the New Testament canon erodes.  (Banning of abortion, for example, is most clearly and directly justified as a Christian dogma by the first century writing termed the Didache.  All other attempts to adduce Biblical authority and justifications tend to be dodges.)  Around 1900 some American religious conservatives realize a retrenching is necessary, that the authority claimed by the Churches over public life from their postApostolic traditions was quite defeated.   They created Fundamentalism, which was/is silly but puts up a collective defensive line all religious conservatives can ultimately retreat behind the at the New Testament, i.e. the authority of Paul over the collective of Christians in American public life.

The run of battle has continued and during the 20th century all American religious conservatives have been unable to sustain the authority of e.g. the Pope or any other religious organization or its writings.  They had to retreat into the authority of Paul.  Paul is heavily about sexual practices and marriages, the priesthood, and methods and arguments that keep congregations of several hundred people from breaking up in disputes.  

And those have been the issues about which the religious part of the Culture War has been.  (There are other American Culture War issues, but they are basically about race/caste and colonial arrangement of social, economic, and political organization of the country.)  The defensive line has not held all that well, ultimately; early defeats of Pauline dogma took fifty years and were on things like divorce and declining authority of the priesthood.  Womens rights and breakups of denominations and individual congregations from the most liberal first to the most conservative last constitute the middle phase.  The only nearly unbroken line of trenches that religious conservatives have left in American public life now is on gay marriage: a slight national majority against it that looks to fade fairly rapidly.  There will be nasty combat around some pillboxes they continue to hold, like attempts to restrict abortion rights in very Red states, but that is dead ender stuff.

They are going to have to retreat, give up on trying to keep (and impose) the authority of Paul and Pauline writings in American public life, fairly soon.  The retreat is necessarily to the appeal, doctrines, and authority of the traditional interpretation of the Gospels and Jesus in American public life.  Which, as can be seen in the case of Democrats this year, is far too great and important to oppose or be neutral about in public.  Though when gay marriage decides and the massed fleeing and retreat of religious conservatives behind those defensive lines and pillboxes happens, no doubt the next great series of disputes begins.

The problem is that the Religious Left is pretty useless, is compromised until they do that retreat.  People like Rick Warren, Jim Wallis, Cizik, and such try to straddle the battlefield of whether Paul's views about sexual behavior and the like are doctrine (conservative) or just opinion (liberal), but it simply doesn't work.  They represent the conflicted and passive, liberal when unpressured but mostly default to conservative when pressured, nature of many/most American Christians about it rather than a true force that exerts itself to deciding the dispute(s).  They have allegiances on both sides of the trenches that are the battleground.

The value of the Religious Left lies at present in their neutrality, their minimal willingness to exert themselves for the Religious Right.  But until the basic theological problem is decided by the conservative oldest parishioners and clergy born before WW2 fading away they will predictably be quite helpless and fairly useless.  I'd say its ten or twelve years until the Religious Left leaders stand before enough of a power vacuum left by retreat/decline of the Religious Right that they can fill it.  Until the present Culture War battlefield of gender and sexual issues lies silent and abandoned, and they can take possession.