MIDLOTHIAN, Va. — The Liberal Women of Chesterfield County did not exist when Representative Dave Brat, propelled by Tea Party-infused energy, shocked the Republican establishment in 2014 and defeated the House majority leader, Eric Cantor, in a primary triumph here that presaged even greater political upheaval two years later.

Now they’re all up in his grill.

Mr. Brat stunned Mr. Cantor, building an army of grass-roots supporters on the ground that was aided in the air by conservative commentators like Laura Ingraham and Mark Levin. Then he made waves of another kind last year, complaining to Republican supporters about women, their health care protests, and their dogged presence in his face.

“The women are in my grill no matter where I go,” he moaned, adding, “They come up — ‘When is your next town hall?’ And believe me, it’s not to give positive input.”

They haven’t gone away. A race that was once considered solidly in the Republican camp is now rated a tossup, and the Liberal Women of Chesterfield County are stirring women to come out in such great numbers that the Democratic nominee, Abigail Spanberger, a well-credentialed former C.I.A. officer, may make history of a different sort in a district that has been deeply Republican for decades.