How and why have attitudes on abortion among Protestant institutions shifted? I use a comparative historical approach and study official abortion stances, archival materials, and periodical articles of the largest and most prominent Mainline Protestant denominations from 1960 to today. I find that Mainline Protestant stances on abortion have shifted dramatically over time, but in strikingly homogenous ways across denominations. In 1960, no Mainline Protestant denomination supported abortion access. During a first wave of shifting from 1966-1972, all denominations suddenly supported expanding abortion rights. During a second wave of shifting from 1988-1992, all denominations stated new hesitations to abortion access. I argue that shifting stances on abortion are connected to how a religious group negotiated shifting ethnic, religious, and political boundaries in the United States since the 1960s. During the first wave of shifting, Mainline Protestants were united by a common enemy: Catholics. During the second wave of shifting, the opponent shifted from Catholics to Evangelical Protestants, who had different political identities and who were sometimes sitting in the Mainline Protestants own pews.