Evangelicals haven't always been part of the pro-life coalition. Prior to Roe v. Wade in 1973, the Southern Baptist Convention passed a resolution supporting abortion in certain circumstances. After Roe allowed any abortion for any reason, evangelicals began to change their stance and with Catholics formed the pro-life coalition we know today. The Washington Post  reports:
The reality of abortion on demand and exposure to the logic of the abortion rights movement led to a fundamental shift in the evangelical conscience. By 1976 the Southern Baptist Convention would declare every abortion to be a “decision to terminate the life of an innocent human being.” Similarly, the large evangelical movement would develop an overwhelming pro-life consensus, seeing abortion as a great moral evil and a threat to the dignity of all human life.