John W. Foust is the Democratic candidate in a competitive race for Virginia’s 10th Congressional District seat. (Melina Mara/The Washington Post)

National Democrats have canceled a multimillion-dollar television ad buy in Northern Virginia’s 10th Congressional District, a blow to Democrat John W. Foust in his race against Republican Barbara J. Comstock for a seat that had been seen as very competitive for much of the year.

A Democrat tracking ad purchases confirmed that about $2.8 million in ad spending is being canceled, with most of the money going instead to protect freshman Rep. Ami Bera (D-Calif.).

“Ad reservations are changing every week, and John Foust is running an aggressive campaign in a tough climate,” David Bergstein, spokesman for the Democratic Congressional Campaign Committee, said in response to the move, which was first reported by Roll Call.

The 10th District leans Republican, but Democrats have been bullish on a seat that has not been competitive for 30 years in a part of Virginia that has become increasingly liberal and diverse. The 10th District is home to the only competitive House race in a state that President Obama won twice and where Democrats swept into all three statewide offices last year, yet where Democrats hold only three congressional districts.

A spokeswoman for the National Republican Congressional Committee called the news “extremely bad” for Foust. “Clearly the DCCC can see what a strong candidate Barbara Comstock is and that Foust doesn’t stand a chance against her,” said Katie Prill.

Foust campaign manager Shaun Daniels said that the situation is not so dire.

“None of the facts of our race have changed,” Daniels said, adding that the DCCC often makes reservations early in a cycle and then moves money around. “Everything here is currently as it was yesterday.”

Last week the DCCC announced that it was drawing down its ad spending in 12 long-shot congressional districts to better protect seats held by Democrats.

The ad wars in Virginia’s 10th District may not be too unevenly matched despite the DCCC’s decision. According to a person tracking ad buys in the district, Democrats are still spending more here, thanks to a much larger commitment from Foust’s own campaign.

Foust himself has reserved $3 million in ad spending, far more than the $226,000 reserved by Comstock’s campaign. House Majority PAC, a super PAC supporting Democrats, is also spending $767,163. On the Republican side, the NRCC has pledged to spend $1.4 million on ads in the district in the next month, while a supportive super PAC has pitched in close to $1 million and the U.S. Chamber of Commerce another $287,000.

Foust, a Fairfax County supervisor, has focused on Comstock’s opposition to abortion in an effort to woo independent female voters. Comstock, a state delegate, has in turn hammered Foust as sexist for questioning whether she’s ever had a “real job.” Several Democrats have privately said that the comment, meant to skewer Comstock’s history as a partisan political operative, was harmful to his campaign.

Comstock has also emphasized her ties to incumbent Frank R. Wolf, a popular congressman retiring after three decades in office, for whom she once served as an aide.

Stretching from McLean to the West Virginia border, the district is rural, urban and suburban. It is middle class in some places and extremely wealthy in others. It has a high proportion of well-educated voters. Its diversity in ethnicity and socioeconomic status makes it in many ways a perfect test case of the messages both parties are sending.

The DCCC has outraised the NRCC so far this cycle but has also been involved in more races. Foust outraised Comstock in previous campaign quarters; both say they raised more than $1 million each between July and September but have yet to release detailed reports.