Smith’s successor as New York governor, Franklin D. Roosevelt, had better political fortunes: He won a landslide presidential victory in 1932 after the economy crashed, and his New Deal incorporated several elements of the socialist agenda, including Social Security and unemployment insurance. But FDR’s victory did little to solidify Democrats’ standing in the city. Passing the New Deal required the cooperation of politicians representing the distressed rural South. An alliance between the interests of the white manufacturing working class and southern segregationists lasted for decades.

Miles away from New Deal negotiations in Washington, however, millions of black Americans were forcing the third inflection point as they moved from the rural South to cities, especially in the North and Midwest. During the Great Migration, from 1900 to 1960, the black percentage of the populations in South Carolina and Florida declined by more than 20 percent. In that same time period, the African American share of Detroit, Cleveland, and Chicago rose from less than 2 percent in each city to more than 20 percent of the population.

Black voters pushed urban Democrats in these northern cities to protect their labor and voting interests.

By the early 1960s, the Democratic Party was an unstable coalition, balancing the support of black urban workers with that of southern segregationists from whom they had fled. With the Civil Rights Act, signed by Lyndon B. Johnson in 1964, Democrats effectively renounced their southern flank. Out of 20 southern Democratic senators, just one—Ralph Yarborough, of Texas—voted in favor of the bill. In 1968, Democrats won less than 10 percent of the once-dependable white southern vote while sweeping the urban manufacturing cores of the Northeast and the Midwest, from Worcester to Wichita.

Only in the past 20 years have dense counties in the South become reliably Democratic, Rodden writes. Since the 1990s, southern cities have become more similar to their northern kin, boasting a similar mix of large companies and explosive growth in college-educated adults. What’s more, the Great Migration has started to reverse itself. In the past few decades, with the deindustrialization of northern manufacturing towns, millions of black families have returned to metros in Texas, Georgia, and North Carolina, bringing their politics with them. Black families on the move have twice pushed Democrats toward becoming a party of the city—first in the North, and then in the South.

Read: Red state, blue city

There is no obvious reason why a 19th-century movement led by Irish Catholic noneducated factory workers should become a 21st-century party for college grads, nonwhite voters, and software developers that defends gay rights, women’s rights, and legalized abortion. But it makes sense if you understand the Democratic Party through the lens of the modern city. Starting in the 1970s through today, Democrats and Republicans have been compelled to take sides on issues that hadn’t previously been politicized. And they have routinely sorted themselves along urban-rural lines, creating a pattern where there was once merely a tendency.