Peggy Noonan remarks on the curious circumstance in which the religion of presidential candidates has become more important in recent years than it has been for the most part in the past.  “In 1968 we were, as now, a religious country,” writes Noonan.  “But when we walked to the polls, we thought we were about to hire a president, not a Bible study teacher.”       
One explanation may lie in the universal-multicultural continuum in which we have been caught for some years now.  A diversity publication from the Association of American Colleges and Universities notes the impossibility of living a life entirely in the autonomous, atomistic, rights-bearing individualism of the universal ideal, and recommends ways in which to realize a greater substantive identity–including race, ethnicity, gender, and sexual orientation, of course — but also religion.  
The substantive American identity of the past, the common civic and cultural understandings that once provided more of a sense of shared peoplehood, are drained out of our society by repeated proclamations of our universal ideals and denial of the need for any particularistic underpinnings.  People then have recourse to the forms of identity that remain to them and begin to assert these forms inappropriately and without balance.  We have seen this with race and ethnicity, of course, and how they have transformed and disassembled the educational experience that once bound Americans together, but it can be true of religion as well.  Religion is part of what informs public life and there is nothing wrong with citizens bringing their religious sensibility to their public participation, but the scrutiny of the candidates’ religion and religiosity of the last few elections goes much further.  Reagan had the enthusiastic support of the religious right but to my knowledge they never demanded to know the exact nature of his beliefs, observance, and private prayer life.
This doesn’t foreclose other explanations, such as emphasis on religion as a reaction against the aggressive secularism and atheism of recent years.