But I think Sharpton and Jackson both have bigger issues with their Christianity from the left than Obama would/will. Maybe the idiocy factor brings the religious factor into play more for those two, and Obama doesn't have to deal with the idiocy factor in the least.
If it weren't for the reborn fanatic currently in office, I don't think Obama (or any other candidate) would have to defend being religious as long as they didn't define themselves and their vision in terms of their religion.
Relgious Affiliation of U.S. Presidents
Episcopalian 11
Presbyterian 10
Methodist 5
Baptist 4
Unitarian 4
Disciples of Christ 3
Dutch Reformed 2
Quaker 2
Congregationalist 2
Catholic 1
Jehovah's Witness 1
TOTAL 42
Source
There isn't an avowed atheist or agnostic on the list, and I don't think the left, any more than the right, expects a candidate to be without religion.