WASHINGTON — From the start, he was a central casting misfire — the dark artist slicing through the capital by electric scooter, a cloak-and-dagger digger better known to former colleagues for scratching his bare belly in plain office view.

In a past career, Glenn R. Simpson had been a reporter’s reporter, tenacious through two decades in journalism, often driving the Washington story of the day — congressional corruption, fund-raising shenanigans, sundry misbehavior — but never becoming it himself. “It’s not news when things go right,” he told a group of students in 1991, describing his craft. “When things go right, it’s boring.”

Mr. Simpson’s life has not been boring for some time now. It has, perhaps inevitably, become news.

As investigators circle President Trump’s administration over ties to Russia during the 2016 campaign, Mr. Simpson, a 53-year-old Wall Street Journal veteran-turned-master of high-dollar research, has arrived at the biggest story of either of his careers, lurching to the center of the Russia-tinged scandal that clouds the presidency.

Mr. Trump knows his work intimately. Mr. Simpson is the man behind an explosive dossier — produced at his firm, Fusion GPS, with a former British spy, Christopher Steele — outlining possible connections between the president, his associates and Russian officials.