Updated

Democrat Joe Courtney's lead in the 2nd Congressional District dropped to 66 votes Monday after officials discovered he was mistakenly given 100 extra votes over Republican Rep. Rob Simmons, an election official said.

"It was human error," said Lebanon election moderator John Bendoraitis. "It was strictly misreading one number on one machine."

The discovery significantly tightens one of the closest congressional races in the nation. Preliminary Election Day returns had Courtney winning by 167 votes out of nearly 250,000 ballots cast.

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It was one of 10 races across the nation that remained unresolved in the days after Election Day, and the only one in which a Democratic challenger had the lead.

Courtney claimed victory on Wednesday and was in Washington Monday to attend orientation sessions. Simmons has not conceded the race.

"We remain highly confident in the results for us and Joe is currently in Washington at the orientation getting ready to take his seat in the 110th Congress," said Courtney spokesman Brian Farber.

Bendoraitis said the error happened when an official on election night read the vote total on one of six machines as 363 votes for Courtney when it should have been 263. With the revised total, Courtney received 1,353 votes in Lebanon, compared with 1,585 for Simmons.

More than 30 of the 65 towns in the district were recounting their votes Monday. Others had already completed their recounts, and the rest must finish by the weekend.

As of Sunday, Courtney held a 166-vote lead.

"These things definitely happen," said state Republican Party Chairman George Gallo. "I still have hope in this stuff."

The recount was triggered because Courtney's unofficial victory margin of 167 votes was less than half of 1 percent and fewer than 2,000 votes.