Beyond his newcomer opponent, Mr. Barr has had to deal with some unexpected issues. The tariffs on bourbon, prompted by the president’s trade war, are wildly unpopular here and could cancel out any benefits from the tax cuts. The Sixth District uses its Bourbon Trail of distilleries as a major tourist attraction, and the industry generates millions in tax revenues.

Farmers have also been hurt. And the largest employer in the district, Toyota, is likely to suffer from Mr. Trump’s tariffs on steel and aluminum.

Mr. Barr said that he has taken his opposition to the tariffs directly to Vice President Mike Pence; the Treasury secretary, Steven Mnuchin; and the commerce secretary, Wilbur Ross, among others — access that he said his Democratic opponent would never have in a Republican administration.

Still, Mr. Barr said he is an enthusiastic backer of the president and would “absolutely” welcome Mr. Trump to campaign for him.

While the midterms will clearly be a referendum on Mr. Trump, the elections will also be a measure of women’s emerging political power. Mr. Barr’s approach of casting his opponent as a reflexive liberal may be more difficult given Ms. McGrath’s military background.

But his deriding her as a “feminist” may be more of a gamble in the #MeToo era. Asked if he considered that term a pejorative, Mr. Barr noted that he has a working spouse, Carol Leavell Barr, and two young daughters whom he featured in an earlier ad.

“I am a strong advocate for equal opportunity, regardless of gender,” he said. “What people around here understand, though, is that feminism used the way my opponent uses it is the politics of entitlement based on an immutable characteristic, the politics of grievance. That is not the way the people of the Sixth District view equal opportunity.”