The modern Republican Party is the party of conservative Christianity and big business--two things so closely identified with the contemporary GOP that we hardly notice the strangeness of the pairing. Legal historian Mary Ziegler traces how the anti-abortion movement helped to forge and later upend this alliance. Beginning with the Supreme Court's landmark decision in Buckley v. Valeo, right-to-lifers fought to gain power in the GOP by changing how campaign spending--and the First Amendment--work. The antiabortion movement helped to revolutionize the rules of money in US politics and convinced conservative voters to fixate on the federal courts. Ultimately, the campaign finance landscape that abortion foes created fueled the GOP's embrace of populism and the rise of Donald Trump. Ziegler offers a surprising new view of the slow drift to extremes in American politics--and explains how it had everything to do with campaign spending.