Kaiman issued a statement saying he had declined Nassau Democratic chairman Jay Jacobs' invitation to participate in the party's executive committee screening of candidates for county executive last night.
"He will not be seeking the Democratic nomination," spokesman Justin Myers said in an email.
Kaiman could not be reached for comment.
Former County Executive Thomas Suozzi and businessman Adam Haber remain in the race for the Democratic nomination to run against Republican County Executive Edward Mangano this fall. Jacobs has been backing Suozzi for months.
Jacobs called Kaiman a "loyal Democrat and a good friend. He certainly is qualified to have done the job but I appreciate his decision and respect it and look forward to working very closely with him as we nominate Tom Suozzi and then elect Tom the next county executive."
Haber said, "I thank Jon for his continued leadership in North Hempstead. As a Democrat I know that our party's primary process will produce the best candidate to face Mangano this fall and take back Nassau County for the middle class."
Mangano spokesman Brian Nevin said, "Regardless of who's the candidate, residents will remember in November that Ed Mangano never raised their property taxes and even cut taxes by repealing the home energy tax." Suozzi as county executive had backed the 2.5 percent tax on home heating fuels as a way to raise $38 million to balance the county budget.
Kaiman announced last month that he was forming a campaign committee to explore a run for county executive this fall. His announcement came a day before Suozzi released a statement that he would run for his old job. Suozzi initially had said he wouldn't run but Jacobs publicly lobbied for him to enter the race.
It was unclear whether Kaiman, a former District Court judge who was elected town supervisor in 2004, would run for re-election this fall. "I have been urging him to consider that," Jacobs said. "I know that he has a passion for North Hempstead. He's dedicated a lot of years to the job and I'm sure he feels he's got more to do."
Democrats will hold their nominating convention for this fall's candidates in late May.