The best analysis I've seen is by church historian Martin Marty who faults Warren, McCain and Obama for "Using God Politically." Here's a quote:
William Lee Miller a half century ago noted that the American founders, friendly to religion as they were, found ways to draw a line of distinction between religion and the civil authority but still found religion useful. Just as there are "water works" and other utilities, religion (and God) became utilities, noted Miller. They would support morality. Fine. But in the world of party politics it is almost impossible to talk about candidates' inner spiritual life without putting them in the place where they give answers that will help them build support or lose it.
When a Jimmy Carter said that as a Baptist he believed in "soul liberty" and held views about human rights that could complicate foreign policy, we were alerted and took that into consideration when voting. Voters knew what they were getting in policy, not in matters of the heart. Ronald Reagan did the same with other policy issues. But if or when either of them or their successors had or has to show how much they love Jesus and how fervently they call upon God, they are asked to step over a line, and they do. Religion is then a "public utility," something which confuses public policy and does authentic religion no favor
No, it is not violating the separation of church and state to ask and answer "soul" questions, and yes, you cannot keep faith and politics apart. But "faith" as it affects policy is one (admittedly tricky) thing; faith as faith cannot be enlarged upon in the public forum without coming close to exploiting religion and making God part of campaign slogans.