Republican vice presidential candidate Sarah Palin was in friendly territory Monday. She rallied a couple thousand supporters in Leesburg, Virginia. The location was just over the border of Virginia’s Fairfax County, into Loudoun County—a place that has picked a Republican for president in every election dating back to 1968.
 

It’s a sign of where this election stands just over a week from Nov. 4. The McCain campaign is spending a good deal of time in places it needs to protect electorally. One sign behind Palin at Monday’s rally read “Keep Virginia Red.”
 

If the McCain campaign is going to do that, Loudoun County is one spot it desperately needs to hang onto. Loudoun has seen major population growth over the past four years, including a large surge in the county’s immigrant population.
 

“Loudoun and Prince William Counties—those are key for McCain,” says Robert Lang, senior fellow at the Brookings Institute and co-director of the Metropolitan Institute at Virginia Tech. “If Obama wins those counties, even if he wins them by half a point, he wins the state.”
 

Lang, along with Brookings fellows Alan Berube and Thomas Sanchez, penned a chapter in “Red, Blue and Purple America,” Ruy Teixeira’s new collection of analysis on the future of election demographics in America. They write on “the new suburban politics,” an analysis of metro voting trends since 2000.
 

The analysis puts Loudoun County into the “emerging suburb” category—counties where at least 25 percent of the population live in an urban area and where growth is accelerating. Like “exurban counties,” the emerging suburbs are spots where George Bush won handily in 2004, and places where Democrats can make major gains by simply keeping GOP margins down.
 

The battle for Republicans is to hang onto emerging suburbs like Loudon and Prince William Counties, and not allow demographic shifts and population growth to turn those spots blue.
 

Take neighboring Fairfax County, which Lang and company term a “mature suburb.” Fairfax voted for a Democratic presidential candidate four years ago for the first time since 1964. The trend accelerated in 2006 and helped elect Virginia Democrat Jim Webb to the U.S. Senate.
 

The crux of their analysis is that these spots are the new fault lines in American politics. If Democrats can improve their numbers even slightly as the geography moves from the urban center to the suburbs and exurbs, the party could make major electoral gains.
 

In 2000 and 2004, the dividing line between Democratic and Republican dominance fell fairly close to the urban centers of most key states. In 2006, that line stretched much further out, reflecting suburban Democratic gains. It also helps explain the Democratic sweep in Congress.
 

If, as most pollsters and many other political observers believe, Loudoun County is currently a toss-up between McCain and Obama, it means that dividing line has the potential to stretch even further out into Republican territory in 2008. Right now that’s a big advantage to the Democrat.
 

Shane D’Aprile is web editor at Politics magazine. sdaprile@politicsmagazine.com