Anna Throne-Holst increased her lead over David Calone to 188 votes Thursday in the Democratic primary in the 1st Congressional District as the Suffolk Board of Elections began counting 1,811 mail-in absentee ballots.

Under the watch of a dozen supporters and attorneys from each campaign, election workers tallied 477 votes for Throne-Holst and 318 for Calone — 44 percent of the outstanding paper ballots.

Former Southampton Supervisor Throne-Holst led Calone, a venture capitalist, by 29 votes on election night. Both campaigns said Thursday it was still too early to draw conclusions.

The count is expected to continue Friday and could resume Monday.

The winner will face freshman Rep. Lee Zeldin (R-Shirley) in November, in what is expected to be one of the most heavily contested congressional elections in the region.

Campaign volunteers armed with laptops and overseen by election attorneys hired by each campaign huddled around the three tables at the Board of Elections’ warehouse in Yaphank.

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The outside of each ballot was inspected by the supporters before it was opened by election workers. The campaigns had a stack of yellow cards to challenge ballots.

As of lunchtime, Calone supporters had challenged three votes, while Throne-Holst challenged six. The campaigns jointly challenged one ballot.

A decision whether the challenged ballots will be counted will be made by the Democratic and Republican Board of Elections commissioner after the other ballots are counted.

Both campaigns grumbled that only three tables had been set up, delaying the final tally and preventing the winner from focusing on Zeldin.

Republican Elections Commissioner Nicolas LaLota said only three Republican employees were available for the count. He noted this was what was used in a close county legislative race last year. “I think the pace of this is very fair,” he said.

By state law, all counting has to be done by one Democratic Board of Elections employee and one Republican.

Calone campaign manager Rahul Kale said they were ready for additional tables to be set up. “The sooner we can get through this process, the better it will be for the eventual Democratic nominee,” Kale said.

Throne-Holst manager Andrew Grunwald said, “We look forward to votes being counted as quickly and accurately as possible so we can focus on the ultimate goal of defeating Lee Zeldin and Donald Trump in November.”