The California Supreme Court may reveal Thursday whether it intends to uphold Proposition 8, and if so, whether an estimated 18,000 same-sex marriages will remain valid, during a high-stakes televised session that has sparked plans for demonstrations throughout the state.

By now, the court already has drafted a decision on the case, with an author and at least three other justices willing to sign it. Oral arguments sometimes result in changes to the draft, but rarely do they change the majority position. The ruling is due in 90 days.

Chief Justice Ronald M. George, who wrote the historic May 15, 2008, decision that gave same-sex couples the right to marry, will be the one to watch during the hearing because he is often in the majority and usually writes the rulings in the most controversial cases.

Most legal analysts expect that the court will garner enough votes to uphold existing marriages but not enough to overturn Proposition 8. The dissenters in May's 4-3 marriage ruling said the decision should be left to the voters.

One conservative constitutional scholar has said that the court could both affirm its historic May 15 ruling giving gays equality and uphold Proposition 8 by requiring the state to use a term other than "marriage" and apply it to all couples, gay and straight.

"The alternatives are for the court to accept Proposition 8 and authorize the people to rewrite the Constitution in a way that undermines a basic principle of equality," said Pepperdine law professor Douglas Kmiec. If the court overturns Proposition 8, "that is the short course toward impeachment."

The court is under intense pressure. Opponents of gay marriage have threatened to mount a campaign to boot justices who vote to overturn the initiative. The last time voters ousted state high court justices was in 1986, when then-Chief Justice Rose Bird and two colleagues lost a retention election.

On the other side, the Legislature has passed two resolutions opposing Proposition 8, and protests are being planned statewide to urge the court to throw out the measure.

Thousands are expected to descend Thursday on the San Francisco Civic Center to watch the hearing live on a giant outdoor screen, just steps from the courtroom where the justices will be prodding lawyers in a jammed courtroom.

"It is one of the most important cases in the history of the California Supreme Court," said Mark Rosenbaum, legal director of the ACLU of Southern California. "The core tenet of our constitutional democracy is that fundamental rights of historically disadvantaged minorities are not dependent on the whim of the majority."

The challenges to the initiative are based on novel legal theories. Gay rights lawyers argue that the measure was an illegal constitutional revision, rather than a more limited amendment. The court has struck down constitutional amendments passed by voters as impermissible revisions only twice in its history, and there are relatively few precedents on the subject.

"While no case forecloses the revision argument, there is no case that really supports it, and most of the cases mildly cut against it," said UC Davis law professor Vikram Amar.

Upholding existing same-sex marriages would be a lower hurdle for the court, Amar and other scholars said.

"There is enough ambiguity in Prop. 8 that the court could easily interpret the measure as not applying to existing marriages," Amar said. "That is a legally plausible interpretation, and it is so clearly the just interpretation that I think getting four votes for that seems easier."

State Atty. Gen. Jerry Brown's office will ask the court to uphold the marriages and strike down the initiative as an illegal repeal of an inalienable right without compelling justification. Brown's argument is even more novel than the revision challenge, which his office said had no merit.

The Proposition 8 case has attracted more friend-of-the-court briefs than the marriage dispute that led to last year's historic ruling -- the previous record-holder. Most of the outside groups that have weighed in have asked the court to overturn the initiative.

Pepperdine's Kmiec said replacing the word "marriage" with another term would both leave intact the court's May 15 ruling and deter a recall campaign that could damage the court as an institution. He said couples could still marry in their religious communities.

That would "restore a religious meaning to a word that is a religious word," he said. Kmiec, a Catholic, said he reluctantly voted for Proposition 8 "because of the instructions of my faith community" but felt "entirely unsatisfied" with the outcome.

"I am not sure Ron George wants to be remembered as the chief justice who denied the principle of fundamental equality," the law professor said. "It is not a legacy we should ask anyone to live with, and it is wholly unnecessary."

George, a moderate Republican, is considered a swing vote on the court and, until the marriage decision, was widely regarded as cautious. Scholars have said the marriage ruling would be pivotal to his legacy on the court.

"It is difficult to imagine, although obviously plausible, that the majority of justices who ruled in the marriage cases would so quickly endorse an undermining of at least a significant portion of their ruling," said Kate Kendell, executive director of the San Francisco-based National Center for Lesbian Rights.

Pepperdine law school Dean Kenneth Starr, hired by the Proposition 8 campaign, will urge the court to uphold the measure and declare that existing same-sex marriages are no longer valid. Benefits, such as inheritance, acquired by couples during their marriages would not be taken away, but couples would have to register as domestic partners to protect their future rights.

"The people ultimately decided," Starr wrote in his final brief in the case. "Under our system of constitutional government, that is the end of the matter."