The California Supreme Court refused to intervene Wednesday in a state commission’s disciplinary proceedings against veteran Contra Costa County Superior Court Judge Bruce Mills, whose lawyer claims the commission has a conflict of interest.

The Commission on Judicial Performance, which has reprimanded Mills three times in the past, filed new allegations against him in October, accusing him of holding an improper private meeting with a prosecutor in a drunken-driving case and, in a second case, trying to deny a defendant good-behavior credits that could reduce his jail term for contempt of court.

The defendant in the second case was Joseph Sweeney, who separately has accused the state commission of covering up judicial misconduct, and has led a campaign that prompted the state auditor to begin the first probe of the commission’s operations.

Citing Sweeney’s involvement in Mills’ disciplinary case, Mills’ lawyer, James A. Murphy, asked the state Supreme Court to remove the commission from the proceedings and appoint an independent panel to review the disciplinary charges.

“Our position is that a reasonably objective person would conclude that the Commission on Judicial Performance has a hard time being fair and impartial with regard to Judge Mills,” Murphy said in an interview.

A judge who heard a case under similar circumstances, and did not step aside, would be subject to discipline, he said.

But the court unanimously denied the request Wednesday, in a one-line order, leaving the case with the commission. The commission has not yet scheduled a hearing, where a panel of hearing officers would decide whether the allegations against Mills are true and, if so, what discipline should be imposed. It has the power to remove a judge from the bench, subject to review by the state’s high court.

Mills, a former prosecutor, was appointed to the bench by Gov. Pete Wilson in 1995 and has been elected by county voters to a succession of six-year terms, most recently in 2014.

The commission privately reprimanded him in 2001 for ignoring a defendant’s request for a lawyer and trying to coerce the defendant to plead guilty. It publicly reprimanded him in 2006 for making what it described as “discourteous” and “demeaning” comments to people appearing in his court. And it reprimanded him again in 2014 for contacting a juvenile court judge who was hearing a case against Mills’ son.

In one of the new allegations, the commission said that after another judge had found Sweeney in contempt of court in a family law case, Mills sentenced Sweeney to 25 days in jail in August 2016 but told him and his lawyer he could get the sentence cut in half for good behavior in jail. But the commission said Mill imposed the sentence without half-time credits, told the sheriff’s office not to reduce Sweeney’s term, and relented only when Sweeney’s lawyer contacted him nine days later.

In the other case, the commission said, while a jury in Mills’ court was deliberating drunken-driving charges in March 2016, the judge met with the prosecutor, talked about his own experience prosecuting such cases, and mentioned data that might be used to help the prosecution.

The jury deadlocked, but Mills did not remove himself from the case until the district attorney’s office disclosed the private conversation, the commission said.

The case is Mills vs. CJP, S245704.

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @egelko