Democratic presidential candidate and former U.S. Secretary of State Hillary Clinton speaks during the 2015 United States Conference of Mayors on June 20, 2015 in San Francisco, California. Clinton commented on the Charlestonmassacre during her speech after the shooting deaths of nine churchgoers at the Emanuel African Methodist Church in , South Carolina. The 83rd Annual Meeting of the U.S. Conference of Mayors runs through June 22.National Journal

Hillary Clinton's impassioned comments about race drew the most attention when she addressed the U.S. Conference of Mayors about the Charleston, South Carolina, tragedy last weekend in San Francisco. But the moment of loudest applause during her remarks pointed to a different divide reshaping American politics.

Hillary Clinton commented on the Charleston massacre during her remarks at the 2015 United States Conference of Mayors on June 20 in San Francisco. (Justin Sullivan/Getty Images)Nearly every mayor in the cavernous ballroom rose when Clinton pledged to revive the cause of "common-sense gun reforms" that deny weapons to "criminals and the violently unstable." As the mayors cheered, it was easy to forget that since Bill Clinton's first term as president, an impenetrable phalanx of resistance from nonurban America has blocked all gun-control measures in Congress. Though gun control retains widespread support in central cities, that hasn't overcome indivisible opposition from congressional Republicans, who almost all represent suburban and rural constituencies, and a few rural Democrats who side with them. Charleston probably won't change that.

Gun control may be the issue that most sharply divides urban from nonurban America. But it is hardly the only one. President Obama enjoys widespread support from big-city mayors, including some Republicans, on most of his key domestic initiatives, from health reform to providing universal preschool. "He is the first urban president since John Kennedy, so it's not a shock his agenda aligns with what we are doing," says Chicago Mayor Rahm Emanuel, Obama's first-term chief of staff. But from Congress to state legislatures, these same ideas face ferocious opposition from suburban and rural Republicans, sometimes joined by the dwindling ranks of rural Democrats.

The priorities of urban and nonurban America may conflict more today than at any point since the 1920s. Back then, rural America—mostly white and heavily evangelical—backed Prohibition and immigration restrictions in a rearguard effort to impose its values on a rising urban America teeming with the ethnic and religious diversity of new immigrants. The cities forging a new America won that round when they coalesced to help elect Franklin Roosevelt in 1932 (and to repeal Prohibition soon after).

The modern Democratic coalition is again overwhelmingly centered on cities. In 2012, Obama won reelection by more than 5 million votes. Yet he won only 690 of America's 3,113 counties—fewer than any winner since 1920. Obama triumphed by dominating America's most populous urban centers and many of its inner suburbs, even as his support withered beyond them.

A map of Congress, or of most state legislatures, would similarly show Democrats consolidating control over urban centers but waning outside them. The same demographic pattern drives both trends. Cities today are largely populated by minorities and by the whites who feel comfortable living amid racial and cultural diversity. That sorting has created a left-leaning urban consensus that allows Democrats to elect mayors in virtually every major city, including many in otherwise red states (like Houston, Phoenix, and Salt Lake City). The flip side is that apart from some white-collar, culturally liberal suburbs (particularly along the coasts), Republicans are routing Democrats among the white voters who have chosen for cultural or economic reasons to live beyond city centers, somewhere on the continuum from suburb to small town.

This alignment has left Democrats strong in the foundation and penthouse of American politics—city hall and the White House. But in between, Republicans are enjoying their greatest strength in Congress and state governments since the 1920s.

The parties increasingly wage their policy struggles from these competing strongholds. After 26 Republican-leaning states, many of them rural, sued to block Obama's executive action providing legal status to undocumented immigrants, 33 cities legally intervened to support him. While the Republican Congress has ignored Obama's call to increase the minimum wage, cities including Los Angeles and Seattle are raising their own. Cities are embracing other Obama priorities Congress has shelved, including expanded preschool (Denver, San Antonio, and Chicago), paid sick leave (Philadelphia), and equal workplace treatment for gay residents (more than 200 cities). Conversely, on issues from the minimum wage to sick leave, some conservative state legislatures have passed laws prohibiting liberal cities from acting.

In decisions under their control, cities are pursuing a historic wave of progressive innovation—often with White House support. But cities continually face frustrations over national policy, especially in the Senate where the Founders' decision allocating two senators to each state combines with the filibuster to magnify rural influence. Nothing better demonstrated that dynamic than the Senate's 2013 vote rejecting universal background checks for gun purchases. If you assign each senator half of their state's population, the 55 senators supporting background checks represented 194 million people, and the 45 opposing it 118 million. Yet by sustaining a filibuster, the minority blocked the bill.

Hillary Clinton wants mayors to charge that hill again, but the last skirmish over gun control actually offered a pointed reminder that on most issues requiring national action, nonurban America holds a veto over urban priorities—and probably will for years ahead.

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