James Walkinshaw, Democrat of Virginia, Wins House Special Election

A Northern Virginia Democrat coasted to victory in a special election on Tuesday for the U.S. House seat left vacant after the death of former Representative Gerald E. Connolly, an expected result that will slightly narrow the party’s deficit in the chamber.

James Walkinshaw, a former chief of staff to Mr. Connolly who won his former boss’s endorsement shortly before he died in May, defeated Stewart Whitson, a Republican lobbyist for a conservative think tank, according to The Associated Press.

Mr. Walkinshaw, 42, who is an elected member of the Fairfax County Board of Supervisors, is the first Democrat to win a House special election since President Trump took office in January. He framed the election as “our first chance to send a Democrat to Congress since Trump took office again.”

The district, most of which sits in Fairfax County across the Potomac River from the capital, has been solid Democratic territory since Mr. Connolly was first elected to Congress in 2008. Mr. Connolly carried at least two-thirds of the vote in each election since 2016.

While the race did not attract much attention or campaign spending, Democrats have sought to use special elections as a measuring stick for enthusiasm during the Trump era. That may be an easier sell in districts where Republican candidates put up more of a fight. Mr. Whitson “has not held any recent campaign events,” the Washington ABC affiliate reported on Sunday.

Mr. Whitson, 45, is a former F.B.I. agent who emerged from a seven-way Republican primary by playing up his allegiance to and affinity for Mr. Trump. In February, he testified before Congress that it should “make the DOGE reforms permanent,” a reference to the Department of Government Efficiency that was once led by Elon Musk, the tech billionaire.

Mr. Connolly’s death, in May from esophageal cancer, left his seat empty during a summer in which Mr. Trump’s assault on the federal work force was particularly acute in Northern Virginia. And it highlighted the tendency of Democratic members of Congress to remain in office well into their advanced years or ill health.

Democrats are expected to claim another House seat this month when Arizona voters choose a replacement for Representative Raúl Grijalva, who died in March. Mr. Grijalva, who was 77, was being treated for cancer. Another House vacancy created by the death of a sitting Democrat is scheduled to be filled in November with a Texas special election to replace Representative Sylvester Turner, who also died in March. He was 70.

Once Mr. Walkinshaw is seated, House Republicans will be able to lose only two of their members to pass legislation in the chamber. That margin would drop to just one if Democrats, as expected, win the coming special elections in Arizona and Texas.