BRISTOW, Va. - If Virginia is the state that could swing the presidential election and Prince William and Loudoun counties are the suburbs that could swing Virginia, then ground zero in the race for the White House might be here in Elizabeth Engbert's neighborhood.

Engbert, 72, lives here in Prince William County and votes Republican. Her neighbors used to vote the same way. But these days Engbert's street has nearly as many campaign yard signs for President Obama as Republican Mitt Romney and her friend across the street with whom she plays dominoes has asked her to stop sending him GOP campaign literature.

"He's got an Obama sticker on his car," Engbert said. "He asked me to stop sending him stuff [against] Obama."

Prince William and Loudoun counties were once islands of Republican red surrounded by the Democratic blue of Fairfax, Arlington and Alexandria. But the two rapidly growing counties have been transformed by a flood of new residents, many of them minority immigrants, and now look more like political tossups.

Four years ago, Obama took 58 percent of the vote in Prince William and 54 percent in Loudoun on his way to becoming first Democratic presidential contender to win Virginia since 1964.

While Romney is battling hard to take Virginia away from Obama this year, and has erased much of the president's early lead in the state, Democrats see Northern Virginia as the region Obama must dominate if he's to replicate his 2008 victory and, ultimately, win a second term.

"Northern Virginia is absolutely pivotal for a Barack Obama victory in Virginia," George Burke, the 11th Congressional District Democratic Chairman, told The Washington Examiner. "No Democrat can win without it."

Obama's support in Northern Virginia has remained relatively steady even as his popularity waned elsewhere in Virginia. The region is home to tens of thousands of federal employees and thousands of others who do business with the government and federal spending during Obama's first term largely protected the area from the worst of the recession. Unemployment in Northern Virginia is consistently below the national average.

Still, Republicans insist that Mitt Romney can cut into Obama's support here, mainly by flipping Prince William and Loudoun. Obama may have won them in 2008, Republicans note, but Republican Bob McDonnell won both a year later on his way to becoming governor.

Meg Desmedt, president of the Bull Run Republican Women's Club, said enthusiasm for Obama has tapered off across the area.

"I think that a lot of people voted for Obama in 2008 because they wanted a change and they were just frustrated," Desmedt told The Examiner. "But a lot of them have now said, wait, this is not what we needed."

Obama and Romney have been making regular appearances in Northern Virginia. Obama brought former President Clinton to Prince William County Saturday to make a last-minute pitch to voters who could decide which way Virginia -- and, ultimately, the race for the White House -- go. But it was a harder sell this time.

Gayle Lawson, a self-employed hairdresser from Nokesville, is among those who voted for Obama in 2008 but now isn't sure who she'll back on Tuesday.

"Maybe it's time for a change," Lawson said. "I'm really undecided. And I'll probably be undecided when I get to the voting booth."