My take on it is I think it's because of Reagan's embracing of the Moral Majority and this strange Reagan lust that ANY Republican can do no wrong while any Democrat must be a heathen because they don't embrace as much religious affiliation as Reagan, Bush 1 & 2 and Nixon did (or claimed to).
Bill Clinton getting a bj and the misguided claim that Obama was a Muslim didn't help matters much, so now evangelicals have poised themselves to believe that Republicans can do no wrong while Democrats are essentially the anti-Christ.
It's gotten so warped that they are willing to support and defend a man and a party that explicitly says and does things against Christ's teachings of loving thy neighbor as thyself and helping those less fortunate than themselves.
When confronted with this hypocrisy, many resort to quoting the US Constitution rather than The Bible as defense claiming that there are no laws in the Constitution citing any wrongdoing.
