What are you basing this on?
Furthermore, you say your estimate is 70-30 McCain. Yet you say that poll (which is basically even) is not far from reality. Well, 70-30 and tied is a pretty big difference. Explain that.
Because I work for what most would categorize as a conservative Christian company. I am sure Barna takes into account Evangelicals who are more liberal than my workplace. I think this is actually one of McCain’s greatest shortcomings and could be his downfall this election. There are many Christians who buy into Obama being a Christian. Governor Palin has helped.
The emergent movement has moved many Christians to the left. To many things like war, green issues, and the like trump homosexuality and abortion.
It drives me crazy but I really don’t think Barna is far off.