A reapportionment plan that Californians will vote on in a special election in December has a good chance of being approved. It not only could switch control of the legislature to the Republicans, but it also could unseat some of the most powerful California Democrats in Congress, leaders of both parties said today.

Democratic Party leaders, faced with an expensive, bruising campaign that is expected to distract many members of the California congressional delegation for the next five months, said only a court challenge filed today may stand in the way of the Republican-sponsored redistricting plan.

"Throw all the rascals out is a very persuasive theme," said California Democratic Party Chairman Peter Kelly.

The new plan would throw many Democratic incumbents, and some Republicans, into the same districts, and would add Republican voters to many Democratic-controlled districts.

This evening, the legislature ended a record 19-day deadlock by sending Republican Gov. George Deukemejian a $27.2 billion budget that he is expected to cut radically and, after a delay, sign the trailer bill that implements the budget. The legislature immediately recessed for a month.

Deukmejian's decision Monday to call a Dec. 13 special election for the Sebastiani initiative, named after the Don Sebastiani, the millionaire state assemblyman who conceived it, sparked the greatest battle yet in an all-out, two-year war over the size and location of state legislative and congressional districts in the nation's most populous state.

California Democrats, already blessed with a 3-to-2 edge in voter registration, drew districts after the 1980 census that maintained their decade-old majorities in both houses of the state legislature and allowed them to swamp the GOP in the 1982 congressional elections.

Democrats gained a net of six congressional seats in California, about a fourth of their total gain nationwide, and have a 28-to-17 advantage in the largest state delegation to the U.S. House of Representatives.

California voters disapproved of the Democratic redistricting plan in a 1982 referendum, but, when they failed to approve a follow-up plan for a special commission to draw new district lines, Democratic legislators passed a second reapportionment scheme not significantly different from the first.

Election experts in both parties said today that the Sebastiani plan, by reconstituting GOP suburban strongholds broken up by the Democrats, provides the opportunity for Republican majorities in the legislature and perhaps even in the congressional delegation by the end of the decade.

Donna Kingwell, an election analyst in the Sacramento office of pollster Lance Tarrance, indicated that the Sebastiani plan would profit from a discernible voter preference for plans described as "fair" and "honest" and generally negative reaction to any mention of the word "incumbent."

The Democratic-drawn congressional districts, fashioned by the late representative Phillip Burton (D-Calif.), who bragged that their intricate and twisting patterns were his contribution "to modern art," do look strange next to the more normally shaped districts in the Sebastiani plan.

Sebastiani and his supporters argue that their plan would split up fewer counties and cities than does the current one. But its effects on powerful congressional incumbents would be considerable.

Sebastiani's plan would cram six Democratic congressional seats in west and southern Los Angeles into four districts. Three congressmen, Reps. Howard L. Berman (D-Calif.), Mel Levine (D-Calif.) and Anthony C. Beilenson (D-Calif.) would find themselves living in the same districts. Rep. Henry A. Waxman (D-Calif.), who may be the most powerful legislator in the state, could be forced to run against a black candidate despite his own solid liberal credentials.

Two leading black congressmen, Rep. Augustus F. Hawkins (D-Calif.) and Julian C. Dixon (D-Calif.), could be forced to run against each other under the Sebastiani plan.

Republicans say the plan preserves the same number of strong black and Latino districts and offers the possibility of increases in minority representation. Democrats counter that the plan dilutes the power of black voters and could severely reduce the number of Latinos in the legislature.

A suit filed in the California Supreme Court by leading Democratic legislators today argued that the Sebastiani plan is unconstitutional under previous state court decisions indicating only one reapportionment plan should be allowed each decade.

"Reapportionment is essentially a destabilizing process," Berman said. "If this is okay, then every two years any politicians who don't like the districting plan can go play the same game."

Democrats said they feared the date of the Dec. 13 special election, between Hanukah and Christmas, would discourage voters who are not accustomed to going to the polls at that time of year. Both sides predicted massive absentee ballot campaigns to try to finesse this problem.