This thesis examines the influence of religion on party realignment in the United States focusing on Catholic voting behavior. A statistical analysis utilizing bivariate analysis and logistical regressions examines if religion and party realignment is an ecumenical trend expanding beyond Evangelicals to Catholics. It measures scientifically the party trends of the Catholic voter. With data pooled from the National Election Studies from 1960 to 2004, it tests the hypothesis that church attending Catholics are realigning over time into the Republican Party both in vote choice and party identification, because of their pro-life position on abortion. The analysis shows that church attending Catholics have dealigned from the Democratic Party over time because of their pro-life position on abortion. The thesis is a model for examining the religion and party realignment question for other traditional Democratic religious denominations such as African-American Evangelicals and Jews.