(c) 2013, Bloomberg News.

(c) 2013, Bloomberg News.

Twenty years ago, half the 12 largest U.S. municipalities had a Republican mayor. When Bill de Blasio takes office in New York on Jan. 1, none will.

As middle-class residents moved out of cities and immigrants and young people replaced them, the party lost its grip on population centers even as it increased control of governor's offices and legislatures. The polarization has pitted urban interests against rural areas and suburbs, denying Republicans a power base.

"The New York election hopefully is somewhat of a wake-up call," said Scott Smith, R, the mayor of Mesa, Ariz., and president of the U.S. Conference of Mayors. "If that doesn't get Republicans on the national level more interested, then it should."

De Blasio's election means that besides New York, there will be Democratic mayors next year in Los Angeles; Chicago; Houston; Philadelphia; Phoenix, Ariz.; San Jose, Calif.; Jacksonville, Fla.; and San Antonio and Dallas, Texas. A runoff will be held in February in San Diego to replace Democratic Mayor Bob Filner, who resigned in August amid charges of sexual harassment.

In New York, Democrats outnumber Republicans more than 6- to-1, yet voters hadn't elected a Democrat since David Dinkins lost to Rudolph Giuliani in 1993. De Blasio, 52, won this month by the biggest margin by a non-incumbent in city history on vow to close the growing gap between rich and poor.

With its concentration of Wall Street professionals and urban poor, New York has one of the highest income disparities among large U.S. cities, according to U.S. Census data. While more than 26 percent of households earned at least $100,000 in 2012, almost a quarter earned less than $25,000.

In all but three of the dozen most populous cities, mayoral elections are nonpartisan and candidates' affiliations don't appear on the ballot. Yet their party is often known to voters.

In Boston's nonpartisan election Nov. 5, both candidates were Democrats. State Rep. Marty Walsh defeated City Councilor John Connolly, in part because opponents painted Connolly as the "Republican" after he attracted donations from that party, Mary Anne Marsh, a Democratic consultant in Boston, said by phone.

Demographics help produce the trend, said Bruce Katz, director of the Metropolitan Policy Program at the Brookings Institution, a nonprofit research organization in Washington.

Racial and ethnic minorities that overwhelmingly support Democrats accounted for 83 percent of U.S. population growth from 2000 to 2008, with most living in the largest metro areas, according to Brookings's 2010 "State of Metropolitan America" report. And cities themselves are growing. Between 1950 and 2010, the proportion of Americans living in urban areas increased to 80.7 percent from 64 percent, according to the U.S. Census Bureau.

Two decades ago, crime was the dominant issue in those places, so law-and-order Republican mayors such as Giuliani in New York and Richard Riordan in Los Angeles were popular, Katz said.

"My motto was, 'Tough enough to turn L.A. around,'" Riordan said in a telephone interview.

Today, the emphasis is on economic matters and equality issues that Democrats champion, Katz said.

"There's this old notion that there's no Republican or Democratic way to pick up the garbage," Katz said in a telephone interview. "That's not entirely true."

Voters selecting mayors care most about who can do the job, and national Republicans are more focused on dogma, said Mayor Greg Ballard in Indianapolis, the most populous U.S. city run by a Republican.

"If they campaign or govern with a basis in ideology, they're going to fail," said Ballard, 59, a former Marine who has supported mass transit and opposed a ban on same- sex marriage. "Being a mayor is about getting things done, and I do wish a few more people understood that."

National Republicans haven't made cities a priority, said Smith of the U.S. Conference of Mayors. While the 2012 Democratic National Convention gave prominent roles to mayors, including a keynote address for San Antonio Mayor Julian Castro, no big-city mayors were featured at the Republican convention.

More Republicans won't become big-city mayors until the party's policies "align a little more closely with what's good for African-Americans, Hispanics and poor whites," Ed Rendell, D, former Philadelphia mayor and Pennsylvania governor, said by phone.

Republicans, however, have increased their share of governorships to 30 from 22 since 2006, according to the Republican Governors Association. They've also taken the mayorships of smaller cities even as they fail to gain traction in the most populous, Kirsten Kukowski, a spokeswoman for the party's national committee in Washington, said by e-mail.

"We are happy with the success we've had in statewide and local elections, but we know we need to find a way to communicate our principles to a wider audience," she said.

The divide is dramatic in places such as Texas, where Houston, Dallas, San Antonio and Austin are run by Democrats while the state as a whole is reliably Republican. All statewide offices are held by Republicans, including Gov. Rick Perry, a 2012 Republican presidential candidate, and the party controls the legislature.

As a result, when Democrats and mayors advocated issues such as higher taxes to support education and expanding Medicaid under President Obama's health-care overhaul, Republicans blocked them, said Mark Jones, chairman of the political-science department at Rice University in Houston.

Democratic mayors are "really left to their own devices," Jones said by phone.

The nation's divide makes consensus difficult on decisions such as whether a state should allow hydraulic fracturing for natural gas, said Joel Kotkin, who teaches about urban studies at Chapman University in Orange, Calif.

"Not only do people have different views, but they don't even see the other people," Kotkin said by phone. "Having a one-party system is just not a very good way to get to the best policy."

_ With assistance from Annie Linskey in Boston.