Only a couple of decades ago, Prince William County was one of the mostly white, far-flung suburbs where Republican candidates went to accumulate the votes to win elections in Virginia.

Since then, Prince William has transformed. Open acreages have given way to townhouses and gated developments, as the county -- about a half-hour south of Washington -- has risen to have the seventh-highest U.S. household income and become the first county in Virginia where minority members make up more than half the population.

If Prince William looks like the future of the country, Democrats have so far developed a much more successful strategy of appealing to that future. On Tuesday, President Obama beat Mitt Romney by almost 15 percentage points in Prince William, nearly doubling George W. Bush's margin over Al Gore in 2000, helping Obama to a surprisingly large victory in Virginia.

He did it not only by winning Hispanic voters, but also by winning strong majorities of the growing number of black voters and of voters younger than 40. A version of his coalition in Virginia -- a combination of minority members, women and younger adults -- also helped Obama win Colorado, Nevada and perhaps Florida, which remained too close to call. He came close in North Carolina, a reliable state for Republican presidential nominees only a few years ago that he narrowly won in 2008.

A demographic wall

The demographic changes in the U.S. electorate have come with striking speed and have left many Republicans, who have not won as many electoral votes as Obama did on Tuesday in 24 years, concerned about their future. The Republicans' so-called Southern strategy, of appealing mostly to white voters, appears to have run into a demographic wall.

"Before, we thought it was an important issue, improving demographically," said Al Cardenas, the chairman of the American Conservative Union. "Now, we know it's an essential issue. You have to ignore reality not to deal with this issue."

The central problem for Republicans is that the Democrats' biggest constituencies are growing. Asian-Americans, for example, made up 3 percent of the electorate, up from 2 percent in 2008, and went for Obama by about 47 percentage points. Republicans increasingly rely on older white voters. And contrary to conventional wisdom, voters do not necessarily grow more conservative as they age; until the last decade, a majority of both younger and older voters both tended to go to the winner of the presidential election.

This year, Obama managed to win despite winning only 39 percent of white voters and 44 percent of voters older than 65, according to exit polls not yet finalized by Edison Research. White men made up only about one-quarter of Obama's voters. In the House next year, for the first time, white men will make up less than half of the Democratic caucus.

The Republican Party "needs messages and policies that appeal to a broader audience," said Mark McKinnon, a former Republican strategist for George W. Bush. "This election proved that trying to expand a shrinking base ain't going to cut it. It's time to put some compassion back in conservatism. The party needs more tolerance, more diversity and a deeper appreciation for the concerns of the middle class."

Calls for outreach

Nothing in politics is permanent, and Republicans may soon find ways to appeal to minority members and younger voters. As Hispanic and Asian voters continue to move up the income scale, for example, more of them may turn skeptical about Democratic calls to raise taxes on the affluent.

And Democrats may yet confront their own challenges once they no longer have Obama and his billion-dollar campaign machine at the top of the ticket. If turnout among blacks, Hispanics and younger voters -- groups that have historically had comparatively low turnout rates -- had declined slightly, Obama might have lost.

But the question for Republicans, people in the party say, is how to improve their image with voters they are already losing in large numbers. Tom Davis, who used to represent Dale City as a Republican member of Congress, said that the problem for his former colleagues goes beyond just Hispanic outreach.

The party's coalition is contracting, not expanding, he says. It has to find a way to broaden its reach, in part by finding more minority and female candidates to run under the GOP banner, he argues. And he said the outreach had to be real: "It's not just putting them into the photo ops at the convention."

Republicans like Davis -- and some inside Romney's campaign -- are quick to point out that the election this week was close. Davis said that it was "not time to panic" for Republicans. But he said Republicans must be honest with themselves about the future. "It is time to sit down practically and say where are we going to add pieces to our coalition," he said. "There just are not enough middle-aged white guys that we can scrape together to win. There's just not enough of them."