New Yorker has a nice article that documents the changing face of politics in the US arising from the homogenisation of the two political parties.
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... 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... 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.
This localised party system has undergone a radical shift in recent times,
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... In the ensuing years (following the civil rights legislation)... segregationists in the South no longer saw the Democratic Party as their natural home... 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...
In the past few decades... Americans have grown less able to name their governor and less likely to vote in local elections. Conversely, they now have much stronger feelings about national figures, like senators or Presidential candidates. If they could choose whether their party got to occupy the White House or the governor’s mansion, most would pick the former. Even the attention of the donor class has nationalized. From 1998 to 2012, the amount of money poured into an average Senate race doubled; the cost of governors’ races barely budged.
And why did this happen,
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... The power of the Presidency had greatly expanded. The national parties had gained vastly more control over state and local subdivisions.
The consequence of this "nationalisation of politics",
According to a recent study by the political scientists Shanto Iyengar and Sean Westwood, Americans may now be more likely to discriminate on the basis of party than on the basis of race: asked to choose between equally qualified scholarship applicants, Democratic and Republican participants alike heavily favored applicants who were identified as belonging to the same political party they did. White participants in the study were much less likely to penalize an applicant for being black than participants of one party were to penalize applicants of the other... People now cast their votes to advance their political ideology, not to get a public job... voters, donors, and activists are much more likely to judge elected officials on whether they pass an ideological purity test than on whether they bring tangible benefits to their districts... Once upon a time, every community in America had its own store with its own local products. Today, chains like Walmart and Home Depot offer the same wares all over the country. The parties, Hopkins believes, have undergone a similar process of homogenization: “Just as an Egg McMuffin is the same in every McDonald’s, America’s two major political parties are increasingly perceived to offer the same choices throughout the country.”...
As Lilliana Mason argues in a sobering new book, “Uncivil Agreement: How Politics Became Our Identity” (Chicago), factors such as class, race, religion, gender, and sexuality used to cut across one another to a significant extent. In an earlier age, a voter might have identified herself as both a conservative and a Presbyterian. Each of these identities predisposed her to have a negative opinion of people who did not belong to the same group. But since there were plenty of non-Presbyterian conservatives, as well as plenty of non-conservative Presbyterians, each of these “cleavages” held the other one in check. In the past decades, though, “partisan, ideological, religious, and racial identities have . . . moved into strong alignment,” Mason writes. Religious communities, for example, are far less politically diverse than they once were: “A single vote can now indicate a person’s partisan preference as well as his or her religion, race, ethnicity, gender, neighborhood and favorite grocery store.” As a result, Mason argues, all those factions have fused into two new mega-identities: Democrat and Republican.