Dante Chinni heads the American Communities Project at American University and writes the Politics Counts blog for the Wall Street Journal. He is the author of Our Patchwork Nation.

Sometime this summer—at long last—Washington D.C.’s mass-transit system, the much-maligned Metro, will open its Silver Line rail branch—a $3 billion project 6 years in the making.

The District’s beleaguered commuters may breathe a sign of relief, but one group won’t be celebrating: the Republican Party. With the Silver Line’s arrival, the GOP may be seeing the final departure of what was once a core part of the party’s hold on Virginia—Loudoun County.


The new Silver Line won’t quite get to Loudoun, at least not in this phase. That’s scheduled to happen in the next part of the Metro’s expansion in 2018. But the new line takes Metro right to the edge of the county, and new stations and the population that follows them seem likely to lock in what has become Loudoun’s move to the political left.

In 2000 the county gave George W. Bush 56 percent of its vote. But by 2012, Barack Obama took it with 51 percent. And in 2013 Virginia’s Democratic Governor Terry McAuliffe carried Loudoun in a three-way race by 5 percentage points. That was four years after Republican Gov. Bob McDonnell carried the county by about 22 points.

Driving Loudoun’s political changes are broader demographic shifts. Since 2000, its population has doubled – going from about 170,000 to almost 350,000. It’s become more diverse – going from about 6 percent Hispanic to almost 13 percent. It’s added more wealthy residents. In short, it’s become more like nearby Washington, D.C.

Like other suburbs around the country, it has urbanized, and in doing so it’s become much more fertile ground for Democrats, as I wrote recently in the Wall Street Journal with Elizabeth Williamson.

Enter the Silver Line. The new Metro stops are certain to bring with them something that the data indicate is an enormous problem for Republicans: greater population density.

Issues and candidates matter in elections, but so do the base populations. (It would take a very special Republican presidential candidate win New York City, for instance.) And, as a rule, the more thickly populated a place is, the more Democratic as well. In 2012, President Obama carried 49 of the 50 most densely populated counties in the United States.

The Silver Line will merge with existing tracks in Arlington County before crossing over into Washington, D.C.

Even if the Silver Line isn’t formally crossing into Loudoun yet, it will be close enough that those living near Dulles, in the nearby Loudoun suburbs of Sterling or Ashburn, might be able to take the train into work. That means those places will likely grow and add different sorts of people—the kinds of people who want to take the train to work.

Already, you can see the signs of change in the county all the way out in Leesburg, where the quaint downtown still exists at the gateway to horse country, but on the edge of town is Village at Leesburg, a mixed-used shopping center where “multifamily residences complement office and retail spaces,” the Village’s website says. It’s like a bit of northwest D.C. transplanted into Loudoun—upscale condos among a walkable set of coffee and yogurt shops, a gym and a Wegmans grocery store.

Census data already suggest that different kinds of people are moving into Loudoun. According to country migration figures, nine of the top 10 counties adding population to Loudoun voted for Obama in 2012. That trend is likely to accelerate post-Silver Line and may move the county from light powder blue to a deep shade of cobalt.

And the data support that idea. An analysis done by software engineer David Troy estimates that when density surpasses roughly 800 people per square mile, counties tend to flip from Republican to Democratic.

Loudoun currently sits at about 605 people per square mile. That’s far lower than neighboring, heavily Democratic Fairfax and its 2,700 per square mile, but it is higher than the state average of 202 per square mile and higher than Loudoun’s year 2000 figure of about 330.

Admittedly, there’s a chicken-and-egg question here. Do densely populated places lure people with a more Democratic sensibility? Or, as places become more densely populated, do people themselves start to lean leftward? The numbers suggest it’s probably a bit of both.

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But in the case of Loudoun and the Silver Line, such poli-sci debates are hardly relevant. There are two true facts. Loudoun is growing more densely populated and more Democratic. And the Silver Line seems poised to make both those facts even truer.

If that happens, the new Metro line won’t just be changing the commuting patterns of the D.C. area, it will be fundamentally changing the national political storyline. And Loudoun, one of those crucial swing counties in a swing state, may become just another reliably blue piece of the Democrats’ electoral map.