Perry Gershon, who lost a close election to Lee Zeldin in the 1st Congressional District race last November but scored major gains for Democrats, will announce Saturday that he’ll run again in 2020.

“I want to get started,” Mr. Gershon said in an interview with The Suffolk Times. “I think we are really off the charts in terms of Democratic excitement … We did a lot right in the race. A lot of people got motivated, and volunteerism was at an all-time high. I want to recapture that energy before it goes away.”

Voters in the district re-elected Mr. Zeldin, a Republican, to a third term in Congress in a hotly contested midterm race that, overall, saw the House of Representatives flip to Democratic control in record turnout nationwide.

On election night, Mr. Zeldin secured approximately 53 percent of the vote — a much closer race than the 16-point win he posted in 2016. Mr. Gershon tallied 46 percent, but within his numbers were major gains for Democrats — spikes that boost his confidence for another race against Mr. Zeldin.

In many ways, there was a silver lining in Mr. Gershon’s loss. After all ballots were counted, he won Southold, a traditionally Republican stronghold, with 5,622 votes to Mr. Zeldin’s 5,292.

Mr. Gershon also won his hometown of East Hampton, along with Shelter Island and Southampton. He lost Riverhead, Brookhaven and the part of Smithtown that is in the district.

“Not winning Riverhead was costly,” Mr. Gershon said. “To win this time, we need to get more votes out of Brookhaven. And I need to do better in Smithtown, where I lost by 7,000 votes.”

A analysis of voting results in the 41 election districts in Southold and Riverhead showed increased support for the Democrat in 39 of them. Democratic support climbed in all of Southold’s 19 election districts. A district in Greenport and another in Southold hamlet, where voters in 2016 supported both Mr. Zeldin and Donald Trump, flipped to Gershon.

Even in Southold districts that went for Mr. Zeldin, such as in Mattituck and Peconic, support for the Democratic candidate increased. Also critical to Mr. Gershon’s view of another race is the fact that, even with fewer Southold voters showing up at the polls for the 2018 midterm than for the 2016 presidential race, he nonetheless received more votes than Democrat Anna Throne-Holst got when she ran against Mr. Zeldin in 2016.

Looking at the political landscape in the district today, Mr. Gershon says he sees a mood that supports him and a cause that’s worth another grueling campaign.

“People liked my story,” he said. “They liked the story of an outsider who wasn’t part of the political establishment, who was for Long Island issues such as the environment and health care.”

Mr. Gershon said he spent $5 million on the campaign, including a primary against four other Democrats. Of that, he said $2 million was his own money.

He said he hopes there will not be a primary challenger, but has heard of at least one other person interested in running. He said he hoped that, based on the closeness of the 2018 race, Washington, D.C., Democrats will get involved on his behalf.

He said the issues are waiting to be addressed: fighting to repeal the SALT cap chief among many. “Mr. Zeldin needs to be held accountable for that,” Mr. Gershon said. “His political party did that to us.”

Mr. Gershon will make his official announcement Saturday at 2 p.m. at Painters’ Restaurant in Brookhaven.

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