This is partly because the Republican party decided to partner with the Christian Right back in the Reagan years (a losing proposition with religion on the decline and the increasing social liberalization since the war years ) and partly because so many of them are mercenary fucks with no principles.
"partner"? You've got to be kidding me. "Suckerpunch" is more like it. Like I said, every army needs cannonfodder. The raison d'etre for evangelical american christianity's entanglement with the republican party was unitary: Deterring abortion. This was dangled as a carrot by the slug-lords of the mammon party, but what happened when the presidency and both the house and the senate were all in republican control? Nothing. They could have passed a law defining the criteria for human life, without the tiniest speedbump of opposition during that 2 years. They did nothing. The only real purpose accomplished by the entanglement of christendom with the political "right" in the u.s. during the past 40 years was turning "evangelical" into a term of oppobrium, and forming an association in the public mind between christianity and the aligned forces of murder, rapine, and pillage.