Ambling along the shore of the Chesapeake Bay on a balmy August morning, Barbara Evans laid out the dilemma she faced as she pondered whom to vote for in November to represent her in Congress. “We’re usually Republicans,” Ms. Evans, a retired business owner, said as the water crept up to her sneakers. “Now with this president, I don’t know.” The war with Iran was weighing heavily on Ms. Evans, the mother of a Marine veteran, as it is for many voters in Virginia’s Second Congressional District, where on any given day, sailors in uniform walk the streets and jet planes dot the skies.

The Navy-heavy district is deeply intertwined with the country’s Atlantic Fleet, which has shouldered a significant share of the branch’s deployments to the Middle East. It is a highly competitive district where Representative Jen Kiggans, a two-term Republican, is battling for re-election in a race that could help decide which party controls the House. Across the United States, G.O.P. lawmakers in battlegrounds face voters’ antipathy toward the war and President Trump’s handling of it. Four years ago, Ms. Kiggans challenged Representative Elaine Luria, a Democrat, by tying her to an unpopular President Joseph R. Biden Jr. and his handling of the Afghanistan withdrawal. The strategy worked: Ms. Kiggans ousted Ms. Luria, where nearly 15 percent of residents are veterans and roughly another 5 percent are on active duty.

The women, both Navy veterans, are up against each other again, but the script has flipped. Now Ms. Kiggans’s support of Mr. Trump and his actions abroad have become potent attack lines for Ms. Luria. Ms. Kiggans has the task of distancing herself from Mr. Trump’s unpopular policies while staying in good favor with a president known to punish members for opposing him. Democrats have reason to hope for an opening. In 2025, Gov. Abigail Spanberger, a Democrat, won the district by nearly eight points, while Mr. Trump won it a year before by a sliver of about 700 votes.

Key to Ms. Luria’s pitch is hitting Ms. Kiggans, a former Navy helicopter pilot, for failing to break with the president on the conflict. The Republican has voted seven times against resolutions directing Mr. Trump to end the war in Iran. Ms. Kiggans followed Mr. Trump in dismissing reports about conditions aboard the Abraham Lincoln. In a statement, Ms. Kiggans said her “top priority will always be the safety and well-being of our service membe...