The justices’ questions to lawyers often reveal how the court is leaning. Legal analysts will be carefully watching Chief Justice Ronald M. George, whose vote often determines whether the conservative or more liberal wing of the court prevails.

The court will need to decide the fate of existing same-sex marriages only if it is prepared to uphold Proposition 8, which many legal analysts believe is likely.

During a three-hour televised hearing this morning, the San Francisco-based high court will examine whether the November ballot measure was an impermissible constitutional revision or a more limited constitutional amendment.

The California Supreme Court will hear arguments today on whether Proposition 8, the anti-gay-marriage initiative, should be upheld and, if so, whether the marriages of an estimated 18,000 same-sex couples should remain valid.

The state high court ruled 4 to 3 on May 15 that same-sex couples should be entitled to marry. George wrote the ruling, which was signed by Justices Joyce L. Kennard, Kathryn Mickle Werdegar and Carlos R. Moreno.

Justices Marvin R. Baxter, Ming W. Chin and Carol A. Corrigan voted against overturning the state’s previous ban on same-sex marriage, arguing that the matter should be left to voters.

After Proposition 8 passed, only Moreno voted to put the measure on hold pending a decision on the legal challenges. Kennard, who usually votes in favor of gay rights, voted against accepting the revision challenge to the proposition but said she would hear arguments over the validity of existing same-sex marriages.

Some legal analysts believe the vote signaled that Kennard did not believe the revision argument would prevail. Without her vote, the court would be unlikely to muster a majority for overturning the measure.

In addition to arguing that Proposition 8 was an illegal constitutional revision, gay rights lawyers contend that it usurped the authority of the courts.

The hearing, scheduled to start at 9 a.m. and end at noon, will be broadcast live on the California Channel and streamed on its website.

The California Constitution can be revised only during a constitutional convention or if two-thirds of the Legislature puts the proposal before voters and they approve it.

Atty. Gen. Jerry Brown’s office will ask the court to overturn the measure on other grounds. Brown has said Proposition 8 must be struck down because it eliminates an inalienable right without compelling reasons.

Both challenges are based on novel legal theories. Gay rights lawyers contend that the measure undermined the Constitution by permitting a simple majority to take away a fundamental right from a constitutionally protected minority that has suffered discrimination.

Legal analysts have said the court is more likely to uphold the existing same-sex marriages than to overturn Proposition 8.

The court received a record number of briefs in the case, with most outside groups calling on the court to overturn the measure.

Opponents of gay marriage have threatened a campaign to remove justices who vote to overturn the measure. If the court upholds the proposition, gay rights activists will consider asking voters to revisit the marriage question in an initiative on the 2010 ballot.

In previous cases, the court has defined a constitutional revision as a change in the fundamental structure or foundational power of state government or one that makes "far-reaching changes in the nature of our basic governmental plan."

Courts in Oregon and Alaska have rejected revision arguments in upholding anti-gay-marriage amendments, but those challenges did not involve marriage rights that the voters repealed.

The California Supreme Court has rejected at least six revision challenges of initiatives, including measures that reinstated the death penalty, changed tax law (Proposition 13) and imposed term limits.

In 1948, the court overturned an initiative as an illegal revision because it made a wide array of changes in the state Constitution. And in 1990, the court struck down an initiative that would have required the courts to apply federal law when determining the rights of criminal defendants.

The relatively few state high court decisions on revision challenges have never directly addressed whether the kind of change made by Proposition 8 could be considered a revision.

Six of the court’s justices were appointed by Republican governors. Moreno is the only Democratic appointee.

--Maura Dolan