The Methodist and Presbyterian churches were the main influence in the United States throughout the first half of the 20th century. However, Catholic influence would grow stronger since the beginning of the 1970s due to the increase of population from Latin American background. This demographic group was majorly Catholic. There was also a sudden increase of more evangelical movements such as the Southern Baptists since 1990.
Well I believe it was in the 1960's when the separation of the church and state became a legal law. This affected religion in schools which in return affected the youth that was growing up during such times. Even though public schooling was meant for an all around education religion was once able to influence the body ( including students and teachers). But in the 70's religion became viewed as something that didn't deal with education and was instead something children should learn at home from parents or the church. This affected politics because politics and the governmental laws go hand in hand.