The Republican rank and file. Having to cater to Tea Partiers, conservatives, libertarians, the Christian Right, and independents makes Republican candidates continually flip flop on a number of issues. The fear of virtually all Republicans is that if they do not cow-tow to all of their voting blocs in some form or another, than a major group will simply stay home and not vote at all. The Democrats seem to not fear this, as they actually have a much diverse range of voters and they will still vote Democrat no matter the contradictions the Democratic candidate brings to the table.
The Republicans need to back off of the social issues such as abortion, gay rights, and stop taking ultra hard-line stances on immigration. Rather than focusing on religion(Christianity), they need to embrace the sciences and emphasize higher education.