On the day Igor Danchenko believes he was betrayed by the U.S. government — when the federal government’s chief law-enforcement officer effectively outed the Russian émigré, then a confidential FBI informant, as the architect of one of the most explosive and controversial documents in American political history — Danchenko was on vacation.

The wiry, voluble man had just arrived at his family’s one-bedroom condo in Ocean City, Maryland, right by the beach and a few hours from their home in northern Virginia. It was shortly after sunset on July 17, 2020, a...