By Dave Warner

PHILADELPHIA (Reuters) - New Jersey state Assembly Republican leader Alex DeCroce collapsed and died in the Capitol after a late night legislative session, police said on Tuesday.

New Jersey Governor Chris Christie immediately postponed his scheduled state-of-the-state message to the legislature on Tuesday to mourn the death of DeCroce, 75, a longtime friend.

New Jersey State Police received a call just minutes before midnight about an unconscious man in a restroom at the Capitol in Trenton, authorities said.

They performed CPR and administered a defibrillator, and DeCroce was pronounced dead by Assemblyman Herb Conaway, a physician.

The Mercer County medical examiner later determined the cause of death was a "heart-related medical issue" and was not considered suspicious, police said.

DeCroce lived in the northern New Jersey town of Parsipanny and represented surrounding Morris County since 1989.

"I lost a dear friend, colleague and mentor," Christie said in a statement. "I have known Alex for nearly 20 years. He helped to give me my start in elective politics in Morris County in 1993," the governor said.

Christie said DeCroce was incredibly hard working, loyal and someone who stood up for his beliefs.

"Alex was a fighter," Christie said, "He knew how to win, and lose, in this chamber gracefully."

DeCroce was a realtor at a firm that bore his name, ERA Gallo & DeCroce.

No new date was announced for Christie's speech.

(Editing by Barbara Goldberg, Peter Bohan and Greg McCune)