WASHINGTON — Vance McAllister, a 39-year-old college dropout and Army veteran, managed to vanquish a formidable Republican opponent backed by the party’s establishment, win a special election in Louisiana’s Fifth District in a 20-point landslide, and score an endorsement from the star of “Duck Dynasty.”

But until Wednesday, Mr. McAllister somehow had never managed to set foot in the nation’s capital — where he is to be sworn in Thursday as the newest Republican member of Congress.

He did not make it to Washington on a class trip, a fund-raising tour, a meeting with party campaign officials, a sightseeing visit or even while making a connection at one of the city’s three major airports.

“When we go on vacation, we go to where it’s nice and warm,” Mr. McAllister said, by way of explanation.