“Who the hell is Ken Dahlberg?” President Richard M. Nixon asked on June 23, 1972, his voice captured on tape in the Oval Office. He would find out soon enough. Without Kenneth H. Dahlberg, Nixon might not have become ensnared in the Watergate scandal and been forced to resign.

Mr. Dahlberg, who died on Tuesday at 94, became the unwitting link between the Nixon re-election campaign and the five men who, only days before Nixon’s remark, were charged with breaking into the Democratic National Committee headquarters at the Watergate complex in Washington.

He had been a fund-raiser for Nixon’s re-election campaign, and his name was on a $25,000 cashier’s check that had been deposited in the bank account of one of the burglars, Bernard L. Barker. The money was to help cover the burglars’ expenses, and Mr. Barker had withdrawn that amount in $100 bills. He was carrying more than $5,000 when he was arrested on June 17.

Bob Woodward, a young reporter for The Washington Post who with Carl Bernstein unraveled the Watergate affair, has called the Dahlberg check the “connective tissue” that turned what they thought was a story about a common crime into one of historic dimensions....