Michael Barone‘s latest column in the New York Post explains how a divisive, landmark Supreme Court ruling from the 1970s fuels today’s partisan chaos.

If Roe v. Wade had never been decided, if the Supreme Court had just declined to hear cases challenging the constitutionality of abortion bans, levels of partisan polarization would not be anything like what we see around us.

That’s my conclusion, and the conclusion of New York Times columnist Ross Douthat. “You’d still have some of the same breakdowns and derangements,” he tweeted in response to Sachs, “but America’s distinctive kind of ideological sorting-out was not by any means predictable from the vantage point of 1970 or 1975.”

It certainly wasn’t. At the time, by legalizing abortion everywhere, Roe seemed to be taking the nation where it was already going. In the five years before the decision was handed down in January 1973, 16 states with 41 percent of the nation’s population had liberalized their abortion laws. Legislatures in other states might have done so later that year.

Most of the nation seemed headed to the abortion regime that has prevailed in most of Europe, where abortion is generally legal, but not as late in pregnancies as required by Roe and subsequent cases. And in a no-Roe America, just as in no-Roe Europe, different places might have had different abortion laws, changeable through the ordinary political process.

The unusually sweeping nature of the opinion in Roe may be due to its having been written by Harry Blackmun, the only one of the 113 Supreme Court justices in history to have devoted most of his prejudicial legal career to being counsel to the Mayo Clinic, defending doctors. Doctors, of course, not pregnant women, were usually the targets of abortion prosecutions.

Far from settling opinion on abortion, Roe made it a national political issue and the focus of national politics.