In recent years there have been growing fissures between Evangelical Christians and the GOP, an association that has been rather strong since about 1980. These fissures are evident especially (but not exclusively) among younger Evangelicals. They still dislike abortion (understatement of the year), but they seem to balance it with other issues that are more characteristic of Democratic concerns: poverty, environment, social justice, and immigration. In addition, a growing number oppose the war in Iraq. They are also sick of people like James Dobson, founder of "Focus on the Family," supposedly being considered a representative voice.
While there are fissures and many Evangelicals are leaving the GOP, not all are joining the Democratic party. Many seem to be caught in the middle and see no party that really aligns with their mixture of concerns. Yet, come November, there may be many Evangelical votes going for Obama. Moreover, more conservative Evangelicals, like Dobson, have had a difficult relationship with the GOP nominee, McCain, and Dobson has even claimed that "if" McCain is the nominee, he's sitting this election out--thank God!
For more, see this article from the Seattle Times.