Shifting Demographics Tilt Presidential Races in American Suburbs

Younger, More Affluent New Residents Are Reshaping the Vote in Metropolitan Regions of Denver, Atlanta, Washington, D.C.

Bill and Heather King and their son, Colt,

This was a pastoral, conservative Washington suburb until a decade ago, when new jobs sprouting in and around the U.S. capital began drawing younger, more affluent people like Bill and Heather King.

Mr. King, a traffic engineer, and Dr. King, a hospital pediatrician, sought to live among other young professionals in a place with the vibrancy of their urban hometowns—qualities they say they found in this former colonial hamlet.

Not long ago, the couple, both 33 years old, might have skipped over Leesburg, the seat of Loudoun County. But the self-described “Democratic-leaning” Kings are among a new crop of residents sinking roots in formerly reliable Republican Party strongholds, reshaping older suburbs in the metropolitan regions of Denver; Columbus, Ohio; Atlanta; Washington and elsewhere.

These neighborhoods—so-called mature suburbs that sprouted in the decades after World War II—have become so densely populated over the past decade that they more closely resemble the big cities nearby. The U.S. census now classifies the counties that contain them as “urban.”

The population of mature suburbs in the U.S. grew to about 60 million in 2010 from about 51 million a decade earlier, according to a Wall Street Journal analysis of census data.

The newer residents look, shop and vote more like urban dwellers than suburbanites of the past. They bring money and diversity to their neighborhoods, supported by jobs in government, academics and technology.

Politically, Democrats see opportunity; Republicans see a challenge. Growth in mature suburbs has helped the Democrats in presidential contests.

George W. Bush, the most recent GOP president, built his two election victories, in part, on broad suburban support. To win the White House in 2016, Republicans must retain their exurban and rural strongholds, while beating back the growing Democratic tide in the suburbs.

From 2000 to 2012, the three mature-suburb counties around Atlanta all grew by double-digits, all saw their incomes rise and all cast a higher percentage of votes for Barack Obama in 2012 than for Al Gore in 2000.

In Franklin County, a mature-suburb county that holds Columbus, Ohio, the population grew by 12% and median household income climbed by about $8,000 over the same period. Democratic voting also surged: Mr. Obama took 60% of the county vote in 2012, compared with 49% for Mr. Gore in 2000.

The population of the two mature-suburb counties around Denver also grew over the same time, along with median household income. In the 2000 contest, Mr. Gore and Mr. Bush split the counties; in 2012, Mr. Obama won both.

Vote totals from the 2012 presidential election show Mr. Obama won the most populous chunk of the suburbs, ceding only the more sparsely-populated exurban reaches to Republican candidate Mitt Romney.

In 2000, the mature-suburb counties split 48% for Messrs. Gore and Bush. In 2012, 52% voted for Mr. Obama and 46% for Mr. Romney.

A Journal analysis of voting patterns in metropolitan regions of Columbus, Denver and Atlanta—which include their mature suburbs—shows that voters went from favoring Mr. Bush or splitting the vote with Mr. Gore in 2000, to favoring Mr. Obama in 2012, by large margins in some cases.

The surge in suburban population isn’t all flight from adjacent cities, which was typical a generation ago. Instead, brains and money from across the U.S. are being lured to more concentrated geographic regions.

“A city is a fun place to be when you’re young,” said Mr. King, formerly of Baltimore, who moved to Leesburg last year with his wife and a baby. The couple is expecting again.

Mr. King works for Loudoun County’s transportation department, where he designs ways to manage traffic and accommodate the Washington subway, which is slated to open a stop here in 2018. His spouse, Dr. King, works at Frederick Memorial Hospital, about 25 minutes away. Together, they earn between $150,000 and $200,000 a year.

“People tend to gravitate toward people with whom they feel comfortable,” Mr. King said, with Loudoun County becoming “definitely more diverse.”

Mr. King voted for Mr. Obama in 2012, and Dr. King voted for Ron Paul. They consider themselves political independents.

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