SAN FRANCISCO — Click. With the highest court in the land holding the remote control, any broadcast of the historic trial of the legal challenge to California’s ban on same-sex marriage just got shut off for good.

The U.S. Supreme Court, sharply divided along its usual conservative-liberal lines, on Wednesday banned the broadcast of the Proposition 8 trial now unfolding in the federal courthouse here, putting a swift end to Chief U.S. District Judge Vaughn Walker’s planned experiment to post the proceedings on his court’s Web site through YouTube.

Walker, at the conclusion of Wednesday’s trial proceedings, sent some signals that there may be broadcast issues yet to be resolved, but even disappointed same-sex marriage backers appeared ready to move on with the central fight over the challenge to the 2008 voter-approved ban on gay nuptials.

“We want to keep the focus on proving our case, winning our case,” said plaintiffs’ attorney Theodore Boutrous Jr., who called the trial “ideal” for public broadcast.

While the ruling stressed that it was not “expressing any view on whether such trials should be broadcast,” it signaled the Supreme Court is not quite ready to embrace cameras in the federal courts, which traditionally have forbid them. The majority concluded that Walker did not follow proper procedures that would have allowed a change in federal court rules to permit cameras in the trial and that the broadcast threatened to harm the fair trial rights of Prop. 8’s defenders.

Justice Stephen Breyer, writing for the four dissenters, criticized the decision, saying there was no harm in broadcasting such an important trial.

Prop. 8 backers had challenged Walker’s order allowing cameras in his courtroom, arguing a number of their witnesses feared they’d be exposed to harassment and intimidation if their testimony were broadcast on the Internet. Walker and the 9th U.S. Circuit Court of Appeals rejected those arguments, but the Supreme Court intervened, even blocking the simulcast of the trial to five other federal courthouses in San Francisco, Portland, Pasadena, Seattle and Brooklyn.

Meanwhile, the trial moved through its third day with the continued testimony of witnesses for the challengers to Prop. 8, which restored California’s ban on same-sex marriage. Same-sex couples have sued to overturn the law, arguing that it violates the equal protection clause of the federal constitution by denying them the same right to marry as heterosexual couples.

There were some sparks during the testimony of Yale University historian George Chauncey, who had been called by the plaintiffs for his expertise on the history of discrimination against gays and lesbians. But it had little to do with his testimony. Instead, the plaintiffs’ lawyers unveiled their first attack on the Prop. 8 campaign through the testimony of one of its leaders, William Tam, who was deposed in December.

A video of that testimony was played for Chauncey in an attempt to undercut the Prop. 8 lawyers’ argument that discrimination against gays is diminishing over time. In both statements and letters, Tam during the campaign equated allowing same-sex marriage to legalizing prostitution and underage sex and warned that allowing gay marriage would encourage youths to become gay, according to the videos of his testimony.

Same-sex marriage advocates plan to call Tam as a witness Friday, and he can be expected to be a reluctant witness. Tam, who intervened to defend Prop. 8 in the federal court case, recently asked Walker to remove him as a defendant because of worries that gay marriage supporters would target him and his family; the judge has not ruled on the issue.

The challenge to Prop. 8 rests in part on the argument that the law was motivated by bias and hostility against gays and lesbians, and the campaign will be a central feature of that argument. Defense lawyers dispute that theory, saying the law was based on a desire to preserve traditional marriage.

The plaintiffs will resume their case today with testimony from Edmund Egan, San Francisco’s chief economist, who is expected to testify on the costs to local governments of denying same-sex couples the right to marry.

The trial will also reveal another side of the gay marriage debate through the testimony of Helen Zia, a lesbian who married her partner before Prop. 8 went into effect.

The California Supreme Court last year refused to invalidate about 18,000 marriage licenses issued to same-sex couples before Prop. 8 was approved by the voters, and Zia is expected to testify on the difference in the lives of those couples.

Contact Howard Mintz at 408-286-0236