My research showed that the collapse of traditional business associations was a turning point in the party’s shift. To better appreciate this, consider what a grass-roots Republican Party looked like 40 years ago.

Factories, meat packing plants and other large industries dominated urban economies. These firms’ owners joined business associations such as the chamber of commerce, which were central to civic and municipal initiatives and doubled — sometimes literally — as Republican Party headquarters. Although such business communities were especially developed in the industrialized Northeast and Midwest, scholars have identified similarly robust business associations in American cities elsewhere.

The business establishment ran the GOP as a business party: against regulation, taxes and unions but mum on hot-button political issues. For instance, a 1976 survey of a Rotary Club in one city I examined found that 90 percent of Rotarians were committed Republicans, but most expressed ambivalence about abortion. Virtually none wanted it to become an important political issue.

Nothing resembling this kind of business establishment remains today.