Given the size, complexity, and sometime intellectual/theological incoherency of American evangelicalism, its probably possible to find evidence for virtually any thesis regarding the political future of American evangelicalism. Howard Fineman finds some who are dissatisfied with the GOP, not for the reasons cited most frequently in recent days (a leftward drift among some evangelicals--see, for example, this piece featuring a triumphalistic Jim Wallis), but because Republicans are insufficiently morally and theologically pure, or perhaps insufficiently conservative across a range of issues.
My bottom line: a religious tendency as capaciously defined as evangelicalism is will inevitably come to be understood as less of a single-minded force in American politics, except with respect to the occasional galvanizing issue. A religion-friendly Democratic Party that accommodated itself to some restrictions on abortion (theyre not there yet) could eat substantially into the Republican advantage, especially if Republican miscues "demobilized" some portion of its religious constituency. In other words, politically evangelicals could come to resemble Roman Catholics--a narrowly divided swing constituency.