In 1974, Mr. Durkin was running for an open seat in the Senate against a Republican congressman, Louis Wyman. On election night, Wyman won by 355 votes out of more than 222,000 ballots cast. After a recount, Mr. Durkin was declared the victor by 10 votes.
A second recount, conducted by a three-member commission led by the Republican state attorney general, Warren B. Rudman, eventually ruled that Wyman won the election by two votes.
The outcome was sharply criticized by Mr. Durkin and other Democrats. Wyman was briefly seated to fill the final four days of the expiring term of the incumbent, Norris Cotton (R), but the Democratic-controlled Senate held up formal recognition of Wyman after the new term began in January 1975.
For months, the Senate Rules Committee debated the election, while examining almost 1,000 contested ballots and holding dozens of roll-call votes. Democrats tried six times to seat Mr. Durkin, only to be blocked by Republican filibusters.
At a hopeless deadlock, the Senate finally declared the New Hampshire seat open, and a special election was called for Sept. 16, 1975. Mr. Durkin won that contest by 27,000 votes.
Mr. Durkin served on the Senate Appropriations and Commerce committees. In 1976, Rudman — the attorney general who had awarded the election to Mr. Durkin’s opponent by two votes — was nominated to be chairman of the Interstate Commerce Commission. Mr. Durkin delayed the confirmation until Rudman withdrew.
Rudman then ran against Mr. Durkin in 1980, defeating him 52 to 48 percent, and served two terms in the Senate.
John Anthony Durkin was born March 29, 1936, in Brookfield, Mass., and was a 1959 graduate of the College of the Holy Cross in Worcester, Mass. After serving in the Navy, he graduated in 1965 from Georgetown University’s law school.
He worked in the Office of the Comptroller of the Currency before moving to New Hampshire in 1966 as an assistant attorney general. He was the state insurance commissioner from 1968 to 1973.
Mr. Durkin had been married to Patricia Durkin and had three children, but complete information about survivors was not immediately available.
He later practiced law in Washington and Manchester, N.H., and in 1990, he lost a bid for the Senate against Robert C. Smith (R).
In 2008, when Al Franken (D) was declared the winner of a disputed Senate race in Minnesota by 312 votes, Mr. Durkin told the Associated Press that he had no desire to relive the prolonged ordeal.
“I’d much rather have read about it,” he said, “than have lived it.”