Attorneys for Kennedy cousin Michael Skakel filed a petition Wednesday asking the U.S. Supreme Court to overturn his murder conviction, saying his due process rights were violated because a statute of limitations had expired.

Skakel, 45, is serving 20 years to life for his 2002 conviction in the 1975 beating death of his Greenwich neighbor, Martha Moxley, when the two were teenagers.

Skakel appealed his conviction to the Connecticut Supreme Court last year, arguing that a five-year statute of limitations, in place in Connecticut at the time Moxley died, had expired when he was charged in 2000. The court unanimously rejected that appeal in January.

The Legislature eliminated Connecticut's statute of limitations for murder in 1976, but the state high court ruled that because the Moxley murder occurred within a five-year window of that legislation, the change applied to his crime.

"Mr. Skakel's constitutional rights as well as the constitutional protections afforded to all citizens are threatened by the Connecticut Supreme Court's ruling," said Theodore B. Olson, a former U.S. Solicitor General who is representing Skakel. "The State of Connecticut's retroactive application to Mr. Skakel of a statute of limitations that the State's highest court had twice held did not apply to cases such as his violated his constitutional right to due process under the law."

Prosecutor Susann Gill declined comment Wednesday, saying she had not seen the petition yet.

If the nation's highest court agrees to hear the case and Skakel prevails on his statute of limitations argument, prosecutors would be barred from trying him again for Moxley's murder, Olson said.

Olson has argued 43 cases before the nation's highest court, including representing George W. Bush in the disputed presidential race of 2000.

The Skakel ruling overturned two of the state Supreme Court's own precedents on the statute of limitations, Olson said in the petition.

"The breadth of the Connecticut Supreme Court's approach to retroactivity is of enormous significance," Olson wrote. "If that approach is permitted, no citizen may rely on a state supreme court's interpretation of its own criminal statutes, no matter how precisely on-point or long-standing or well-settled the prior interpretation. Such a proposition is so far-reaching that this court's review is warranted."

The Connecticut ruling is at odds with decisions by the U.S. Supreme Court as well as federal appeals courts, Olson argued.

Skakel, a nephew of Ethel Kennedy, also has a petition for a new trial pending in Superior Court in Stamford. That petition is based on a claim by Gitano "Tony" Bryant implicating two of his friends in Moxley's murder.