“I was hoping you fellows wouldn’t ask me about that,” said Alexander P. Butterfield, a former White House aide summoned before the staff of the Senate Watergate committee on July 13, 1973.

His questioner had alighted on a matter that would electrify the investigation of corruption and cover-up in President Richard M. Nixon’s administration. It involved a wiretapping — not the attempt by Nixon campaign operatives to eavesdrop on the Democratic national headquarters at the Watergate building in 1972, but the president’s secret bugging of his own office conversations.