Shortly before the 1960 Democratic primary in West Virginia, a close ally of John F. Kennedy’s asked Raymond Chafin, the Party chairman in Logan County, how much it would cost to buy his support. “About thirty-five,” Chafin said, hoping for a windfall of thirty-five hundred dollars. Meeting Kennedy operatives at a local airstrip, he was greeted with a nice surprise: thirty-five thousand dollars in cash.

As promised, Chafin used his control over the local Party machine to help deliver the state to the junior senator from Massachusetts. “The Kennedys were well aware of our brand of politics,” he said years later. “I guess it was their brand, too.”

For much of the twentieth century, the real power in American politics rested not with U.S. representatives or senators but with the governors, mayors, and assemblymen who controlled local purse strings. In many cases, men like Chafin got people elected to Congress in order to reward them for years of loyal service or to rid themselves of ambitious rivals, but national politics was of comparatively little importance. “The politicians who were crucial to the operation of the organization normally stayed home,” one scholar of the period observed.

At the federal level, the two parties resembled loose associations of disparate interests rather than ideologically cohesive movements. They had few resources and virtually no means of insuring ideological discipline among their members. Many Democrats were more conservative than many Republicans.

All of that had real advantages: Congress was, for much of the past century, a place of remarkable comity, where politicians routinely struck compromises on public spending or judicial appointments. Even as Americans found themselves deeply divided on everything from foreign policy to rock and roll, high politics was relatively free of acrimony.

It was also, however, very difficult for ordinary voters to make their voices heard. West Virginia is sometimes touted as the place where Kennedy overcame the biggest obstacle to his candidacy by proving that religious bigotry was no match for his charm. But only fifteen states and the District of Columbia held primaries in 1960, and their outcome was merely advisory. Lyndon B. Johnson, Kennedy’s most serious rival for the Democratic nomination, did not bother entering any of them.

The parties’ lack of ideological definition also made it difficult for citizens to vote their conscience. A liberal who strongly opposed segregation may, for example, have wholeheartedly supported Kennedy. But in voting for him in the general election she would also have voted for a Vice-Presidential nominee, Johnson, who had, as late as 1947, denounced an anti-lynching bill as “a farce and a sham—an effort to set up a police state in the guise of liberty.” (Although Johnson finally backed a civil-rights act in 1957, he allowed amendments that appeased segregationists by rendering it largely unenforceable.) So long as America’s main political parties remained pragmatic associations of local interests, socially progressive Democrats in the North were yoked to segregationist Democrats in the South. Neither Democrats nor Republicans consistently fought to end Jim Crow. The relative lack of partisanship in postwar politics was purchased at the price of violent exclusion.

Assessing the twin problems of organizational weakness and ideological incoherence, a 1950 report by the American Political Science Association sought to turn the loose political federations into something that more closely resembled today’s unified parties. Democrats and Republicans, some of the nation’s most eminent scholars argued, needed to “provide the electorate with a proper range of choice between alternatives of action.” To that end, each party’s candidate was to be determined in a “national presidential primary,” and leaders in Washington were to be given “additional means of dealing with rebellious and disloyal state organizations.” To fix the problems of American government, the scholars believed, politics had to become more national and party platforms more clearly distinguished.

Almost seven decades later, their wish has come true. As Daniel J. Hopkins, a political scientist at the University of Pennsylvania, chronicles in a new book, “The Increasingly United States” (Chicago), American politics has become thoroughly nationalized: voters pay vastly more attention to what is going on in Washington, D.C., than to what’s going on in their own town or state. The Democratic and the Republican Parties have become much more homogeneous, offering largely the same ideological profile in Alabama as they do in Vermont. In each election, Americans now face a choice between two clearly demarcated alternatives of action. The medicine prescribed by the American Political Science Association all those years ago has been taken; the question is whether the patient can survive its side effects.

For the first five days after Kennedy was shot, a mourning nation wondered whether his agenda could possibly outlast him. Even key members of the Cabinet doubted whether Johnson, hastily sworn in as the thirty-sixth President of the United States aboard the airplane on which his predecessor had landed in Dallas three hours earlier, would follow through on civil-rights legislation. But when Johnson addressed a joint session of Congress on November 27, 1963, he threw down the gauntlet to Southern Democrats. “No memorial oration or eulogy could more eloquently honor President Kennedy’s memory,” he said, to their horror, “than the earliest possible passage of the civil-rights bill for which he fought so long.”

In the ensuing years, Jim Crow finally came to an end—and so did the highly local party system that had prevailed, in one form or another, since the Civil War. Segregationists in the South no longer saw the Democratic Party as their natural home. In 1968, many of them supported the third-party candidacy of George Wallace, formerly the Democratic governor of Alabama. During the following decades, conservative Democrats slowly gravitated toward the Republican Party, and the Democratic Party, for the first time in its history, became liberal on both social and economic issues: across the nation, Democrats now stood for at least some modicum of wealth redistribution and racial integration.

Republicans underwent a similar transformation, adopting a militant preference for free markets and low taxes while opposing abortion and gay rights. At the same time, they set out to capitalize on the electoral opportunity presented by the schism in the Democratic Party. Starting with Richard Nixon, every Republican candidate who took the White House employed some form of what had been named, in a deceptively genteel turn of phrase, the Southern Strategy.

As the ambitious civil-rights legislation of the nineteen-sixties realigned America’s political parties, a host of deeper structural changes redirected citizens’ attention toward the capital. Thanks to the postwar boom, public jobs came to look less attractive than private ones, weakening the power wielded by local party bosses. More recent changes in the media have also played an important role. Local papers and radio stations, once the country’s dominant sources of information, brought together national, state, and municipal news; as a result, Americans who were primarily interested in what was going on in Washington still learned a lot about their home towns. Today, voters increasingly get their news from broadcast networks and cable channels, or from social-media sites and online publications, which are less likely to require them to pay attention to their city hall or state capitol.

As early as the nineteen-eighties, political scientists were noting that the nature of American politics was changing in fundamental ways. The power of the Presidency had greatly expanded. The national parties had gained vastly more control over state and local subdivisions. “In the sense that Paris is the capital of France,” the political scientist William M. Lunch observed in 1987, “Washington is becoming the capital of the United States.”