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‘JUDAS’ MORGY BACKS CY OVER AIDE

Retiring Manhattan District Attorney Robert Morgenthau will support trial lawyer and family friend Cyrus Vance Jr. as his successor, sources revealed yesterday – incurring the wrath of his top deputy.

Morgenthau’s longtime second-in-command, Chief Assistant Daniel Castleman – who had let it be known he planned to run for the office, and as late as Friday had expected to get his boss’ support – resigned after Morgy gave him the news Monday, sources said.

“Dan thinks Morgenthau screwed him,” said a source close to Castleman, insisting Morgenthau at various points had promised Castleman his backing.

Another source called Morgenthau a “Judas.”

Now, the sources said, Castleman – who couldn’t raise money while still working in the DA’s office – isn’t expected to run.

Castleman didn’t respond to requests for comment yesterday.

Several sources said Morgenthau had at times led both Castleman and Vance – who worked for the DA in the 1980s and has a hefty war chest, a large group of connected advisers and family ties to the DA – to believe they would win his support.

Castleman has worked under Morgenthau for 29 years and was a driving force behind some of the office’s most high-profile investigations.

Sources said Morgenthau, 89, told Castleman more than a year ago he’d get the DA’s support – but only if he could raise his public profile and links to the community enough to be a viable candidate against former Judge Leslie Crocker Snyder.

Morgenthau is intent on keeping her out of the office after her 2005 primary race against him in which she made hay of his age and said new blood was needed.

Many Morgenthau supporters also believed he needed to make his preferred successor known early on to clear the potentially crowded field. But another well-financed candidate, gun control advocate and former assistant DA Richard Aborn, made clear last night that he has no plans to bow out.

“A clarified field will make it easier for voters to see that there is one true progressive in this race: Richard Aborn,” said spokesman Alex Navarro-McKay.

laura.italiano@nypost.com