For more than 30 years, religious conservatives have been a loyal and, crucially, a predictable voting bloc for the Republicans. This resulted in a lasting deal for Republican candidates: Pledge fealty to the “Christian nation,” promise to ban abortion and (at one time) same-sex marriage, and evangelicals will form an essential and reliable segment of your voting base.

Evangelicals have forgiven past favored candidates for their sins. Ronald Reagan deviated from the movement’s standards on divorce, but he was adept at using religious language, such as “shining city upon the hill.” George W. Bush had an imperfect past, but was redeemed, in evangelical eyes, through religious salvation. In 2004, as 78 percent of white evangelicals voted for George W. Bush, they made up 36 percent of the Bush vote.

Enter Mr. Trump, the candidate who prides himself on unpredictability. His lack of familiarity with the Bible has been a frequent target of ridicule. He publicly declared himself to be against abortion in 2011, when he first toyed with a run for president. While Mitt Romney’s change of heart on abortion — which he dutifully and repeatedly addressed — gave him the flip-flopper label, Mr. Trump has so far repelled any similar branding.

Mr. Trump also gambled on two dynamics that were already altering the evangelical-Republican relationship. First, he recognized how the Supreme Court’s 2015 decision legalizing same-sex marriage marked the end of the road for one of the religious right’s major issues. He declined, for the most part, to weigh in on the religious right’s new formulation on gay rights, which is to frame it as an issue of religious freedom for conservative Christians.

Mr. Trump has frequently proclaimed that when he is president it will be acceptable to start “saying ‘Merry Christmas’ again!” as if the so-called war on Christmas is the sum total of the movement’s religious freedom concerns.