WOODBRIDGE, Va. — Corey Stewart stands at the end of a long driveway that leads back in time, to his 18th century plantation manor hidden in woods behind a modern housing development. Mr. Stewart, the Republican Senate nominee from Virginia, treats the brick home like a living museum, complete with buttons from Redcoats, a Civil War soldier’s belt buckle and a room dedicated to George and Martha Washington, who were once visitors.

Both Mr. Stewart and his opponent, Senator Tim Kaine, were born in Minnesota, which makes it all the more unusual that Mr. Stewart has styled himself as a champion of the Confederacy and its statues, and, as he puts it, “taking back our heritage.”

This has made him a popular figure with white nationalists, much to the horror of many Virginia Republicans. While Mr. Stewart has disavowed some on the extreme right, interviews with dozens of his friends, colleagues, supporters and fellow Republicans yielded a portrait of a political opportunist eager to engage the coarsest racial fringes of his party to advance his Trumpian appeal.

Some white nationalists volunteer for Mr. Stewart’s campaign, and several of his aides and advisers have used racist or anti-Muslim language, or maintained links to outspoken racists like Jason Kessler, the organizer of last year’s violent rally in Charlottesville, Va. Mr. Stewart has not distanced himself from those aides.