Gay rights advocates asked California's highest court Friday to keep off the November ballot a citizens' initiative that would again ban same-sex marriage.

Lawyers for Equality California filed a petition arguing that the proposed amendment to the California Constitution should be invalidated because its impact was not made clear to the millions of voters who signed petitions to qualify the measure before the state Supreme Court legalized same-sex unions.

"This court has recognized that gay and lesbian couples have a fundamental right to marry and, as of June 16, such couples have been getting married across the state," the petition states.

"Rather than effecting 'no change' in existing California law, the proposed initiative would dramatically change existing law by taking that fundamental right away and inscribing discrimination based on a suspect classification into our state Constitution."

The petition also claims the so-called California Marriage Protection Act should be disqualified because it would revise, rather than amend, the state Constitution by altering its fundamental guarantee of equality for all _ in essence writing a law the state high court has already found unconstitutional into the constitution.

"If enacted, it would alter the underlying principles on which the California Constitution is based and make far-reaching changes in the nature of our basic government plan, by severely compromising the core constitutional principle of equal citizenship (and) ... by destroying the courts' quintessential power and role of protecting minorities," it states.

Unlike a constitutional amendment that can be approved by voters, a constitutional revision requires convening a Constitutional convention or the appointment of a commission to recommend changes to the Legislature and voters, according to the petition submitted by same-sex marriage supporters.

"For good reason, there's a strict process for making revisions to our Constitution, and it's more involved than simply collecting petition signatures," said attorney Stephen Bomse in a statement posted on the Web site of the National Center for Lesbian Rights, which filed the request on behalf of Equality California.

"That process is in place to safeguard our basic form of government, especially the most basic principle of equal protection of the laws."

Lawyers from the National Center for Lesbian Rights, who filed the request on behalf of Equality California, did not return calls for comment late Friday.

The petition names Secretary of State Debra Bowen and the measure's sponsors, a coalition of religious and social conservative groups called ProtectMarriage.com, as defendants.

Since Bowen's office already has certified the amendment for the fall election, a spokeswoman says she can remove it only through a court order.

"She has a ministerial duty to certify any initiative when they qualify through the petition process, and she can't remove an initiative without a judge's order," said Kate Folmar, a spokeswoman for the secretary of state.

The Arizona-based Alliance Defense Fund, which represents the measure's sponsors, called the petition a desperate move it would fight.

"This is just another attempt to force a radical political agenda upon the people of California," said Defense Fund senior counsel Glen Lavy. "The opponents of marriage are willing to use any means necessary to impose their will."

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Associated Press writer Paul Elias contributed to this report.

(This version CORRECTS name of secretary of state spokeswoman to Folmar, instead of Folmer)