NEW HAVEN, Conn. — Prosecutors filed an appeal Friday seeking to reinstate Kennedy cousin Michael Skakel’s conviction on a charge of fatally bludgeoning neighbor Martha Moxley with a golf club in 1975, when they were both 15.

In asking the state Supreme Court to reverse a ruling that Skakel’s attorney failed to adequately represent him at the 2002 trial, prosecutors are in the position of defending the actions of a lawyer who once opposed them.

Judge Thomas Bishop granted Skakel, the 53-year-old nephew of Robert F. Kennedy’s widow, Ethel, a new trial last year, saying he likely would have been acquitted if the defense focused more on his brother Thomas Skakel.

Thomas Skakel was an early suspect in the case because he was the last person seen with Moxley. But prosecutors say highlighting Thomas Skakel’s relationship with Moxley would have bolstered their argument that Michael Skakel killed her in a jealous rage. Michael Skakel had admitted to two women that he was aware his brother had sexual contact with the victim the night of the murder and told one woman that is what triggered it, prosecutors wrote.

“To highlight Tommy Skakel’s relationship with Martha would play directly into the state’s hand,” prosecutors wrote. Skakel’s trial attorney, Michael Sherman, “would have been foolish to emphasize the very thing that triggered (Skakel’s) rage: Tommy’s amorous relationship with Martha.”

Skakel’s current attorney, Hubert Santos, has said Thomas Skakel’s encounter with Martha could have occurred as little as 10 minutes before the killing in Greenwich, though another medical examiner said the crime could have occurred later.

Sherman has defended his work. He said there was a lack of evidence to focus on Thomas Skakel and that he had a better case against another early suspect that formed a key part of his defense.

Bishop rejected that claim, saying Thomas Skakel changed his account of the night of the murder years later.

Thomas Skakel admitted he had a sexual encounter with the victim, and the defense could have argued that what could have started as a consensual encounter “may have turned terribly bad,” the judge wrote, noting that the defense only needed to cast doubt that Michael Skakel killed Moxley.

Prosecutors say there was no evidence to suggest what happened between Thomas Skakel and the victim would have caused him to lose his temper.

Michael Skakel was freed from prison last year after the ruling.