WASHINGTON — For a while now, Senator Mark R. Warner has taken to thinking like a Russian.

The Virginia Democrat is losing himself in a book about the Romanovs, eager to absorb the country’s theories of war, teasing staff members for insufficient knowledge of Tolstoy and Nabokov. He has been sending messages over the encrypted app Signal, just to be safe.

Then there was the ruckus outside Mr. Warner’s window late one recent afternoon, a curious thud against the building beside a giant American flag.

“That may just be the F.S.B.,” Mr. Warner said flatly, referring to Russia’s primary intelligence and security agency. He appeared to be kidding. Probably.

Such is the head space of Capitol Hill’s top Democrat investigating ties between President Trump’s orbit and Russian intelligence — a former rising star in the party, frozen in the ascent for years now.