U.S. District Judge Vaughn Walker will step down from the bench at the end of this year.

Walker is best known for ruling California's gay marriage ban, Proposition 8, which defines marriage as a heterosexual union in the California Constitution, in violation of the U.S. Constitution.

The ruling was handed down in August after Walker conducted a 13-day trial in January. Walker wrote that the ban, approved narrowly by voters in November 2008 after the California Supreme Court legalized gay marriage, violated the constitutional rights of gay men and lesbians.

The initiative's sponsors, ProtectMarriage.com, a coalition of social conservative groups that includes the Church of Jesus Christ of Latter-day Saints (the Mormons), the California Catholic Conference and various evangelical churches, defended the law in court after government officials refused, and have appealed Walker's ruling to the Ninth U.S. Circuit Court of Appeals in San Francisco. Oral arguments are scheduled in December.

The 66-year-old Walker gave no reason for his decision, but a colleague told the San Francisco Chronicle that he did not believe Walker's retirement was connected to his marriage ruling.

He is not being “hounded out” by critics, U.S. District Judge Charles Breyer said.

Proposition 8 proponents have blasted the ruling and attacked Walker personally. One brief filed in the appeal takes issue with Walker's rumored sexuality.

“If the allegation that Judge Walker is a homosexual is true, [then] he has a personal interest in the outcome of the trial” and should have recused himself, Robert Wooten wrote in a friend of the court brief.

ProtectMarriage.com has called Walker “egregiously selective and one-sided” in their court filings.

In a statement titled “Got Bias?” Brian Brown, executive director of the National Organization for Marriage (NOM), the nation's most vociferous opponent of gay marriage, accused Walker of “egregious and damaging” bias.

“He's been an amazingly biased and one-sided force throughout this trial, far more akin to an activist than a neutral referee,” Brown said.

Walker was appointed to the court by President George H. W. Bush in 1990 and began serving as chief judge in 2004.

In a letter to President Obama announcing his retirement,Walker said, “ Concluding 21 years of judicial service, I leave the bench with the highest respect and regard for the federal judiciary, its judges and their staff and the essential role they fulfill in our constitutional system.”