As recently as 1984, abortion was not a deeply partisan issue.

“The difference in support for the pro-choice position was a mere six percentage points,” Alan Abramowitz, a political scientist at Emory University, told me by email. “40 percent of Democratic identifiers were pro-life, while 39 percent were pro-choice. Among Republican identifiers, 33 percent were pro-choice, 45 percent were pro-life and 22 percent were in the middle.”

By 2020, Abramowitz continued,

73 percent of Democratic identifiers took the pro-choice position, while only 17 percent took the pro-life position, with 10 percent in the middle. Among Republicans, 60 percent took the pro-life position while 25 percent took the pro-choice position and 15 percent were in the middle. The difference in support for the pro-choice position was 48 percentage points.

This split was even wider, 59 points, among “strong partisans, the group most likely to vote in primary elections,” Abramowitz said.

Crucially, Abramowitz pointed out, opinions on abortion are also closely connected with racial attitudes:

Whites who score high on measures of racial resentment and racial grievance are far more likely to support strict limits on abortion than whites who score low on these measures. This is part of a larger picture in which racial attitudes are increasingly linked with opinions on a wide range of disparate issues including social welfare issues, gun control, immigration and even climate change. The fact that opinions on all of these issues are now closely interconnected and connected with racial attitudes is a key factor in the deep polarization within the electorate that contributes to high levels of straight ticket voting and a declining proportion of swing voters.

Some of the scholars and journalists studying the evolving role of abortion in American politics make the case that key leaders of the conservative movement in the late 1970s and early 1980s — among them Richard Viguerie, Paul Weyrich, Phyllis Schlafly and Jerry Falwell Sr. — were seeking to expand their base beyond those opposed to the civil rights movement. According to this argument, conservative strategists settled on a concerted effort to politicize abortion in part because it dodged the race issue and offered the opportunity to unify conservative Catholics and Evangelicals.

“The anti-abortion movement has been remarkably successful at convincing observers that the positions individuals take on the abortion issue always follow in a deductive way from their supposed moral principles. They don’t,” Katherine Stewart, the author of the 2019 book “The Power Worshipers, wrote in an email.

In 1978, the hostile reaction to an I.R.S. proposal to impose taxes on churches running segregated private schools (“seg academies” for the children of white Southerners seeking to avoid federally mandated school integration orders) provided the opportunity to mobilize born again and evangelical parishioners through the creation of the Moral Majority. As Stewart argues, Viguerie, Weyrich and others on the right were determined to find an issue that could bring together a much larger constituency:

As Weyrich understood, building a new movement around the burning issue of defending the tax advantages of racist schools wasn’t going to be a viable strategy on the national stage. “Stop the tax on segregation” just wasn’t going to inspire the kind of broad-based conservative counterrevolution that Weyrich envisioned.

After long and contentious debate, conservative strategists came to a consensus, Stewart writes: “They landed upon the one surprising word that would supply the key to the political puzzle of the age: ‘abortion.’”