When legislative leaders of both parties reached agreement on a redistricting plan in 2001, they expected that it would eliminate, or at least minimize, interparty competition for seats - and they were mostly correct.

The plan proved to be 97 percent effective in 2002 as voters in just three districts, two in the Assembly and one in the Senate, thwarted their designated partisan ownership by electing Republicans. But that was a bit of a shock to Democratic leaders.

They could be in for another shock this year. There are a half-dozen Assembly districts now held by Democrats that could go the other way this year, although the more realistic Republican hope is for a two-or three-seat gain in the house, now controlled by Democrats, 48-32.

"Timing is everything in politics," Kevin McCarthy, the new Assembly GOP leader, said Tuesday, ticking off the districts in which Republicans will seek gains.

Declining Democratic registration, coupled with the dramatic recall of Democratic Gov. Gray Davis and election of Republican Arnold Schwarzenegger as governor, are creating the opportunity for GOP gains. Republican strategists are targeting districts where Democratic registration has dropped since the redistricting plan was adopted, and whose voters favored the recall and Schwarzenegger. McCarthy said the overall pitch to voters will be to "finish the job you asked Arnold to do."

Democratic strategists acknowledge they have some vulnerability in Assembly races this year but are hoping for a big Democratic presidential vote that will carry over into legislative contests.

Underlying the Democrats' peril is a continuing decline in their share of voter registration. The party once claimed nearly 60 percent of the state's voters but dropped below 50 percent about 1990 and has since declined to 43 percent, its lowest level in many decades. Republicans appear to have stabilized around 35 percent, so the gap between the two is also the smallest in decades.

The Assembly's most vulnerable Democrat may be Nicole Parra, who defeated Republican Dean Gardner by just 265 votes in 2002 - even though Republican leaders refused to invest heavily in Gardner's campaign. Parra's 30th District, which sprawls over farms of the southern San Joaquin Valley, typifies the pro-GOP trend in voter registration. Just since 2002, its Democratic registration margin has dropped by 5 percentage points, and Gardner is running again.

The Democrats' decline has been even more dramatic in the adjacent 31st District, where Democratic incumbent Sarah Reyes is being forced out by term limits. The 18-point registration advantage that Reyes rode to re-election in 2002 now is just 10 points - although Democrats are still favored to retain the seat because they have a strong candidate in Fresno County Supervisor Juan Arambula. To the north, in the Stockton-Tracy area, Democratic Assemblywoman Barbara Matthews could be vulnerable because of her narrow voting margins and an ever-tighter registration gap, but her 17th District is now on the secondary target list.

Two of the targeted districts are along the Southern California coast, where Republicans have ceded much ground in recent elections. Long Beach Democrat Alan Lowenthal is termed out in the 54th District, and with a narrowing registration margin, the district shapes up as a shootout between Democratic state Sen. Betty Karnette and former Republican Assemblyman and Congressman Steve Kuykendall. And in San Diego County's 76th District, now held by Democrat Christine Kehoe, former Republican Assemblywoman Tricia Hunter will vie with the survivor of a tough Democratic primary. The 76th District's Democratic registration has now fallen below 40 percent.

The year's sleeper race could occur in an affluent Silicon Valley area that Republicans once dominated but that has gone Democratic in recent elections. Assemblyman Joe Simitian is taking a shot at the Senate, and while Democrats still hold a 12 percentage-point registration advantage in the 21st District, Republicans have a dream candidate in Steve Poizner, a very wealthy high-tech entrepreneur who later became a classroom teacher, while Democrats have a sharp primary battle to settle who will face Poizner.