"Mr. Sirhan’s assassination of Senator Kennedy is among the most notorious crimes in American history," Newsom wrote in an op-ed published in the L.A. Times on Thursday.
"After decades in prison, he has failed to address the deficiencies that led him to assassinate Senator Kennedy. Mr. Sirhan lacks the insight that would prevent him from making the same types of dangerous decisions he made in the past."
Newsom said he has idolized Robert F. Kennedy throughout his adult life.
A two-person state parole panel recommended Sirhan be granted parole after two of Robert F. Kennedy's children advocated for it.
Robert F. Kennedy was killed about five years after his brother, President John F. Kennedy, was assassinated in Dallas, Texas. Robert F. Kennedy campaigned to end the Vietnam War and was considered a top candidate for president.