I recently read the book, The End of White Christian America by Robert Jones. He's the CEO of PRRI, a institution in Washington that researches public attitudes toward religion. The book traces the history of political involvement in politics by White Christians and their institutions. He covers the Mainline and Evangelical streams as two parts of the same whole. and sees the same demographic and political pressures facing them both; the two streams are following the same trajectory of waning numbers and influence, with the Mainline about 20 years ahead of evangelical Christianity in terms of timeline.
The two issues he traces in detail are gay marriage and race. He shows how White Christianity was on the wrong side of both of these issues, and that the attitude - and the laws - of the country have moved faster and farther than White Christianity wanted to go.