A couple of decades ago, Prince William County was one of the mostly white, somewhat rural, far-flung suburbs where Republican candidates went to accumulate the votes to win elections in Virginia.
Since then, Prince William has been transformed. Open tracts have given way to town houses and gated developments, as the county — about a half-hour south of Washington — has risen to have the seventh-highest household income in the country and has become the first county in Virginia where minorities 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 Mr. 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 Asian-American voters and of voters under age 40. A version of his coalition in Virginia — a combination of minorities, women and younger adults — also helped Mr. 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.
The demographic changes in the American electorate have come with striking speed and have left many Republicans, who have not won as many electoral votes as Mr. Obama did on Tuesday in 24 years, concerned about their future. The Republicans' 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 Mr. Obama by about 47 percentage points. Republicans increasingly rely on older white voters. And contrary to much 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, Mr. Obama managed to win a second term despite winning only 39 percent of white voters and 44 percent of voters older than 65, according to exit polls not yet finalized conducted by Edison Research. White men made up only about one-quarter of Mr. Obama's voters. In the House of Representatives 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 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."
Nothing in politics is permanent, and Republicans may soon find ways to appeal to minorities 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 the Democrats may yet confront their own demographic challenges once they no longer have Mr. Obama and his billion-dollar campaign machine at the top of the ticket, guaranteeing record-breaking turnout among his new Democratic coalition. If turnout among blacks, Hispanics and younger voters — groups that have historically had comparatively low turnout rates — had declined slightly, Mr. Obama might have lost.
But the immediate question for Republicans, people in the party say, is how to improve their image with voters they are already losing in large numbers.
"You don't have to sell out on the issues and suddenly take on the Democratic position on taxes to win the black vote or the Latino vote or the women vote," said Corey Stewart, a Republican who is chairman of the Board of County Supervisors in Prince William. "But you do have to modulate your tone."
Mr. Stewart, who is running for lieutenant governor next year, drew some criticism in 2007 by pushing for local crackdowns by the police on illegal immigrants. That has cost him support among many Hispanic voters in the county, but he says it helped him politically among blacks who felt threatened economically by the surge of newcomers.
"The changes are stark," he said. "The minority population is increasing, and the white population is stagnant."
Mr. Stewart said he had spent much time in the county's minority areas and contrasted his political success with the failure of Mr. Romney, whose only planned visit to Prince William was in the western town of Haymarket, a wealthy, white part of the county.
"He did not go into the minority areas," Mr. Stewart said. "They didn't go into the areas where they didn't feel comfortable. They tended to go to areas where they already had their votes, in heavily white areas."
In Prince William, as elsewhere, the biggest challenge for Republicans may be among Hispanic voters, given their numbers. Mr. Obama's victories in Colorado, Nevada and Virginia came in part because Hispanics turned out in droves and voted Democratic. In Colorado, 14 percent of the voters were Hispanic, and Mr. Obama won three-fourths of them. In Florida, Hispanic voters were almost one-fifth of the electorate, and Mr. Obama won about three-fifths of them.
Mr. Cardenas, a former chairman of the Florida Republican Party and a loyal supporter of Mr. Romney's, says his party would never earn their support until it found a new to address of illegal immigration.
"We need to check off that box; we need to get immigration reform done in 2013," he said. "We need to show that Republicans are willing to sit at the table and reach a compromise that is in keeping with what the Hispanic community wants and needs."
Even that issue brings risks, though, because any immigration bill that passed in 2013 would be signed by and associated with a Democratic president. The harder challenge for Republicans will be developing proposals that minority and younger voters associate with the party — and support.
In Prince William, the Hispanic population tripled from 2000 to 2010, much of it along the Route 1 corridor in Dale City. But 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 Republican banner, Mr. Davis 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 Mr. Davis — and some inside Mr. Romney's campaign — are quick to point out that the election this week was close, not a blowout. Mr. 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."