It began as the half-baked idea of a very rich young state assemblyman whom many considered to be a legislative loon.

Six months and 700,000 petition signatures later, the California Supreme Court this week takes up the Sebastiani initiative, which if enacted could rend the state political map and leave chaos in its place.

Described by its backers as a good-government measure that would erase the gross, partisan gerrymandering of legislative and congressional districts by the Democratic Party two years ago, the redistricting plan created by Sonoma County Republican Don Sebastiani, 30, could return both the state assembly and state senate to Republican control for the first time in a decade.

It also could drastically cut the 28-to-17 advantage Democrats enjoy in the state delegation in the U.S. House and threaten the seats of some of the most influential Californians in Congress, including Democrats Henry A. Waxman, Augustus F. Hawkins, Mel Levine and Anthony C. Beilenson and Republican Bobbi Fiedler.

Briefs filed with the state high court show that every major Democratic Party interest group in the state, including organized labor, women, blacks, Hispanics and supporters of Israel, has organized its own legal team to argue against holding the Dec. 13 statewide vote on the initiative that Gov. George Deukmejian called July 18.

With Republicans generally more eager or able than Democrats to get to the polls during a busy holiday season, one opposing brief said, "there is every indication that the initiative will pass." Democrats and their allies view the court, with a solid majority of Democratic appointees, as the best hope for killing what they see as a monster in its infancy.

Attorneys for assembly Democrats led by Speaker Willie L. Brown Jr., who would lose his powerful post if the initiative passes, argue that the state constitution prohibits more than one redistricting plan a decade.

"Reapportionment is, in short, a serious and troubling transition to be undertaken only upon the highest demands of equality at the polls," their brief argues. "It is not a partisan playground in the interim years, open to whatever group has the votes in Sacramento or the money for direct mail."

If the court allows the special election, attorneys for the Mexican-American Legal Defense and Educational Fund argue, "the floodgates will be completely opened and every statewide primary, general and special election between now and the completion of the 1990 census will include countless, equally incomprehensible initiative redistricting proposals based solely upon the strength of the signatures of 5 percent of the state's electorate."

Attorneys for Sebastiani and his allies reply that the "once-a-decade" rule applies only if there is "an existing and valid reapportionment plan."

State Democrats created a plan in 1982, but the voters threw it out. A plan to turn reapportionment over to a special commission also died at the polls, allowing Democrats in the state legislature, with cooperation from Republicans whose seats were protected, to pass a second similarly gerrymandered plan.

Sebastiani's plan would split up far fewer cities and counties than the current districts do.

It would also adjust the number of registered Democrats and Republicans in each district to ensure closer elections. GOP tacticians want to avoid another election like 1982, when Republican candidates for Congress drew 48 percent of the vote, but won 37 percent of the seats.

Democratic legal briefs do not defend the snake-like boundaries fashioned by their redistricting genius, the late representative Phillip Burton (D-Calif.), who sardonically called his work "my contribution to modern art."

Instead, the briefs focus on the unfairness and potential unconstitutionality of changing the tried-and-true process of having redistricting controlled by the majority party in the legislature.

Noting that the initiative process has never been used to draw new district lines, Sebastiani's Democratic opponents argue that:

* Sebastiani's proposal violates the state constitution's single-subject rule by putting congressional and state legislative boundaries in the same initiative. Two prominent Jewish Republicans said in one brief that they want more GOP representatives in the legislature, but do not want to threaten the seats of pro-Israeli congressmen, like Waxman and Levine, who are Democrats.

* Populations of Sebastiani congressional districts would vary by as much as 1,500, compared with 287 for current districts, thus weakening implementation of federal "one-man, one-vote" requirements. Republicans dispute those figures.

* About 827,000 people would not be able to vote for state senator until 1986 because of changes in district numbers under the Sebastiani plan.

* The special election would cost $15 million in state and local funds that are not budgeted, which would make their appropriation and expenditure unconstitutional.

* The initiative unconstitutionally would void the legislature's power to redistrict the state after the 1990 census.

* Female, black and Hispanic representatives would lose seats under the Sebastiani plan. Sebastiani argues that opportunities for all, particularly blacks, might increase, yet says little about the Democratic complaint that minorities would be under-represented in the special election.

The brief prepared by the National Organization for Women and others said that "women voters . . . are disproportionately burdened by the holidays," particularly if they are sales clerks or working mothers.

The state supreme court has been dominated for years by justices appointed by Democratic governors Edmund G. (Pat) Brown and his son, Edmund G. (Jerry) Brown Jr.

But the court has refrained from ruling on other ballot measures supported by Republicans, including a 1982 reapportionment referendum, until voters had approved them.

Sebastiani's attorneys argue that "to prevent the people from even considering the initiative measure would represent an unprecedented judicial intrusion into what is essentially now a political controversy."