The California Supreme Court announced today that it will rule Tuesday on the constitutionality of Proposition 8, the November ballot measure that resurrected a ban on same-sex marriage.

The ruling, which will be posted at 10 a.m., will also determine whether an estimated 18,000 same-sex marriages will continue to be recognized by the state.

Gay rights lawyers have argued that the ballot measure was an illegal constitutional revision rather than a more limited amendment. Atty. Gen. Jerry Brown urged the court to reject the measure on different grounds. He contended the proposition was unconstitutional because it took away an inalienable right without compelling justification.

Chief Justice Ronald M. George and Justice Joyce L. Kennard will cast key votes in the case. They were part of the four-judge majority that gave gays the right to marry last May, but both indicated at oral argument that they were not persuaded the measure was unconstitutional.

The court’s majority decision should be revealed in the first or second page of the ruling and reiterated in its last paragraph. Separate concurring and dissenting opinions follow.

Counting votes may be tricky because the court is dealing with three different legal issues: the revision challenge, the attorney general’s challenge and the fate of existing same-sex marriages.

The court’s vote on whether Proposition 8 is an impermissible revision, for example, will probably differ from its vote on whether existing marriages should continue to be recognized by the state.

Justices who disagree with the majority file dissents. If they agree with only part of the majority decision, they file an opinion called a partial concurrence and dissent.

During oral argument in March, every justice expressed support for upholding existing marriages. Justice Carlos M. Moreno indicated he believed Proposition 8 was an illegal revision, indicating he would dissent on that question

Moreno might be joined by Justice Kathryn Mickle Werdegar, a former civil rights lawyer who stressed the court was dealing with a novel legal question. Werdegar, however, did not join Moreno in voting to put the measure on hold pending the court’s ruling.

-- Maura Dolan