"But relatively few of the evangelicals who have moved away from the Republican Party have become Democratic, most have become independents," he said.
Green, a noted expert on religion and politics who also teaches at the University of Akron in Ohio, said there were opportunities for both sides.
He saw an opportunity for Obama to woo younger evangelicals drawn by his message of change while McCain could also aim for independent and more centrist evangelicals.
McCain is regarded with suspicion by many of the more conservative evangelicals who regard him as soft on their "core issues" such as their opposition to gay marriage.
At the same time, more centrist evangelicals have warmed to him because he combines opposition to abortion rights with concern for issues such as climate change which they see as a vital part of a wider biblical agenda.
The mainline Protestant churches may not be as vocal on "culture war" issues such as abortion and gay marriage which often figure in US elections but they have sharp partisan divides in their pews.
The survey found members of such denominations almost evenly divided with 41 per cent lining up with the Republican Party and 43 per cent with the Democratic Party.
The Pew survey also found that Mormons are the most staunchly Republican group with 65 per cent of those surveyed identifying with the party.
Members of historically black churches were the most Democratic at 77 per cent. Atheists and agnostics are also strongly Democratic at 65 and 62 per cent respectively.
Jews are largely in the Democratic camp at 66 per cent with only 24 per cent indicating a Republican affiliation.
Members of the Catholic Church - another key "battleground faith" which accounts for almost a quarter of the U.S. population - lean Democratic but like evangelicals are not overwhelmingly in the thrall of either party.
The survey found that 48 per cent of adult US Catholics leaned toward or identified with the Democratic Party while 33 per cent tilted Republican.
