Jon Kaiman, saying “I’m all in,” announced his candidacy for retiring Rep. Steve Israel’s Third District seat Sunday and will give up his paid and unpaid state posts to campaign full-time for the job.

“I’m announcing now, declaring my candidacy,” said Kaiman, former North Hempstead supervisor, in a telephone interview, “and in coming days, I will open a committee and separate from my state job and NIFA.”

Another Democratic would-be contender, Robert Zimmerman, a major party fundraiser and Democratic National Committee member from Great Neck, said Sunday he would not run for the seat.

Kaiman, 53, of Great Neck, said he will give up his $153,000-a-year job as a special adviser to Gov. Andrew M. Cuomo for Sandy storm relief, as well as his unpaid post as chairman of the Nassau Interim Finance Authority within the next week to 10 days.

His decision comes only days after a crowded field of Democratic contenders, including Kaiman, made their pitches for the nomination before the party leaders of Nassau, Suffolk and Queens.

Zimmerman, 61, a co-founder of Zimmerman/Edelson Inc., also had met with community and party leaders.

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He said in a statement Sunday that “it has become very clear to me over these past couple of weeks that my responsibilities to family members, my firm and clients preclude me at this time from planning a campaign.”

Zimmerman added: “As disappointed as I am about this decision, many Democrats have come forward who will serve our congressional district with distinction. I look forward to assisting one of those candidates.”

Washington sources have floated the name of current North Hempstead Supervisor Judi Bosworth as a potential candidate even though she earlier expressed no interest in the race and has made no comment since her name surfaced.

Kaiman said the party screening had nothing to do with his decision, but he felt he had to take steps forward for the June 26 primary. He estimated he would have to raise $1 million for the primary.

“I can’t base my decision on what other people are doing,” he said. “I’m ready and prepared to do this in a meaningful and successful way.”

Kaiman said he is strongly pro-choice, supports Planned Parenthood and wants to rebuild this country’s strong relationship with Israel. He believes in having secure borders but favors a path for citizenship for immigrants. He also said he favors increased efforts for renewable energy.

Jay Jacobs, Nassau Democratic chairman, said he was aware that Kaiman was contemplating the move and is not surprised Kaiman is leaving the state jobs because Cuomo does not permit aides to run for office while on the state payroll. “I think he’ll be a strong contender,” he said.

Richard Schaffer, Suffolk Democratic chairman, said Kaiman’s decision only increases the prospect for a Democratic primary. “There’s a now very remote chance we won’t have a primary,” he said. “But with a dozen contenders I was never under any illusion we would not have a primary.”

“Jon Kaiman is a great guy, as fine public servant and a personal friend,” said Tom Suozzi, former Nassau county executive, who is also considering as congressional run. “I’m about shaking things up to solve problems on behalf of the people . . . and Jon may view himself the same way.”