California congressional seats don't fall vacant very often, so when one becomes available, aspiring legislators come out of the woodwork.

That's especially true since a status quo-oriented redistricting scheme, enacted by the Legislature two years ago, made challenging an incumbent member of Congress almost impossible.

As it happens, however, two Central Valley congressmen whose districts were made challenge-resistant, Republican Doug Ose of Sacramento and Democrat Calvin Dooley of Hanford, opted to retire this year. And their retirements have touched off hard-fought primary campaigns that frame the ideological schisms of both parties.

State Sen. Rico Oller, whose suburb-foothill district overlaps much of Ose's 3rd Congressional District, was the first out of the blocks, but Dan Lungren, a former Long Beach congressman who relocated to Roseville and was elected state attorney general in 1990, quickly followed. The Oller-Lungren duel was shaping up as a fierce battle - with Lungren pushed into the unaccustomed role of relative moderate - when Ose's sister, Mary, suddenly joined the fray.

Mary Ose is clearly the least conservative of the three, having been a registered Democrat before her brother was elected to Congress in 1998 and having contributed money to Democratic campaigns. But the Oses are very wealthy land developers and have pumped much family money into her race, matching the money that Oller has raised from his own considerable fortune and from conservative groups. Although he appears to be favored by House GOP leaders, Lungren - who is not personally wealthy - is feeling the financial squeeze. His fund raising is further hampered by Congressman Ose's clout with would-be national contributors and resentment among some Republicans at the lackluster campaign that Lungren waged after winning the 1998 GOP nomination for governor.

Lungren's inability to match his foes' money leaves him vulnerable to their negative advertisements.

The 3rd Congressional District's overwhelming Republican registration means that next Tuesday's winner is virtually assured of election in November - a condition underscored by the lack of a first-rank Democratic candidate. Oller was openly jubilant that Ose jumped into the contest because it could mean a split among less-conservative Republican voters, thus making his conservative base more potent. And if Oller does win, it would be a gain for the growing bloc of conservative Northern California congressmen aligned with Rep. John Doolittle.

The 20th Congressional District, which runs down the San Joaquin Valley from Fresno to Bakersfield, is a different arena. Dooley was in danger of losing the seat until redistricting added Democratic-leaning urban precincts in Fresno and Bakersfield to what had been a rural district and boosted its Democratic registration margin to 13 percentage points. Nevertheless, Dooley opted out this year and his chief of staff, Lisa Quigley, opted in - but she finds herself in an increasingly nasty duel with Jim Costa, who represented much of the area in the Assembly and state Senate for 24 years until being forced out of the Legislature by term limits.

The Quigley-Costa shootout is clearly ideological. Costa was one of the Legislature's most conservative Democrats, with very close ties to agribusiness, while Quigley is allied with more liberal elements of the party, including labor unions. Each is accusing the other of being the tool of special interests, and while Quigley isn't publicly mentioning Costa's 1986 arrest for soliciting a prostitute in Sacramento (although one of her polls asked voters about it), it's still an issue with some voters.

Unlike the 3rd Congressional District, the Democratic winner in the 20th Congressional District is not guaranteed a win in the fall. The district's Democratic registration has been falling - down four percentage points in the last two years - while Republican strength has been growing, reflecting a recent trend seen throughout the Central Valley. And that gives the Republicans a fair shot at taking a seat away from Democrats, especially if Quigley, the more liberal of the two Democrats, pulls out a primary win. Republicans have a viable candidate, state Sen. Roy Ashburn, waiting in the wings while Quigley and Costa joust.