The closest Senate election happened in the 1974 New Hampshire race between Republican Louis Wyman and Democrat John Durkin. The contest lasted eight months and came down to a margin of victory of just two votes.
After the 1974 resignation of President Richard Nixon, Republicans were not looking too good. On Election Day, the first results showed Wyman won the race by 355 votes, but Durkin quickly asked for a recount.
The recount showed Durkin beat Wyman, but only by eight votes, prompting Wyman to demand another recount. Once the ballots were recounted, it showed that Wyman won the race by two votes!
It wasn’t over though, because John Durkin petitioned the Senate. The full Senate took up the case on January 14, 1975 and worked to examine the questionable ballots.
The Senate debated this issue over the next six weeks, but Wyman proposed a new election, which Durkin agreed to. The election had a record breaking turnout, electing Durkin to the U.S. Senate, where he served until 1980.