LOS ANGELES » California’s congressional delegation, the largest and most influential in the nation, is undergoing a major upheaval, the result of reapportionment and retirements, threatening the state’s influence in Washington next year and forcing members to scramble to withstand what is emerging as a generational wave.

A quarter of the state’s 53-member delegation to Washington could be newcomers in the new Congress, analysts said, the result of at least six members retiring and strong contests in another 10 districts. By contrast, only one seat changed hands between parties in the course of 255 congressional elections in California over the past 10 years.

The potential scope of the upheaval, easily the largest anywhere, is sinking in with every new announcement of a retirement. While some state leaders hail the change as a sign of a healthy democracy — "There’s a lot to be said about mixing it up generationally, to have a constant invigoration of Congress with new fresh eyes and fresh voices," said Nancy Pelosi, the House Democratic leader — it has raised concern among officials at a time when the state is reeling from the economic downturn and is counting on Washington for assistance.

As it is, California faces an already unsympathetic Congress as it seeks legislative support on extending a subway line from downtown Los Angeles to Santa Monica, as well as financial support for building a high-speed rail line from Los Angeles to San Francisco. More worrisome, state officials said, California will not have the clout it once had to withstand what are shaping up to be huge cuts in federal spending in coming years as Congress and the White House try to deal with the deficit.

The development is overshadowing what had initially attracted the most attention in the redistricting process here: that Democrats, who now control 34 seats, stand to gain two to four new seats.

While the full impact of redistricting is beginning to clarify in California, to the benefit of Democrats, the national battle over the process — done every decade in all the states, based on the most recent census — continues to churn. There are several court battles over it, particularly in Texas, which at 32 members has the next biggest delegation and where elections have been put on hold as the courts rule on conflicting maps.

The main focus of campaign strategists has not been whether the final national map will benefit one party over another — there has been little doubt that Republicans, by virtue of having gained control of many statehouses in 2010, would have the edge — but rather, how much of an advantage the Republicans would have.

The new political landscape in California, at least, is evidence of the far-reaching implications of the state adopting a nonpartisan redistricting process. After years of allowing state lawmakers to determine congressional district lines, for 2012, an independent commission, created by voters in a referendum, drew districts without an eye to protecting incumbents. The commission was not even supposed to take into account where it was that incumbent lawmakers lived.

The move has been hailed by outside groups who say government has ossified. Yet from California’s perspective, there is a price to be paid for this. The process in effect purges more senior incumbents at a time when other states continue to use redistricting processes that protect incumbents.

"The problem with a commission in one state and not the others is that the House of Representatives still relies to a great deal on a seniority system," said Rep. Henry A. Waxman, a California Democrat not threatened by the wave. "And it puts us at a disadvantage if the lines are drawn without regard to protecting some of the senior and most important members of the delegation."

Some of the state’s most senior representatives are returning home, among them Rep. Jerry Lewis, the dean of the House Republican delegation, who took office in 1979. Two of the delegation’s more powerful Democrats — Howard L. Berman and Brad Sherman — are engaged in a bitter primary after the redistricting process threw them into the same district. They have, between them, 45 years of seniority in the House.

David Wasserman, the House editor of The Cook Political Report, a nonpartisan political newsletter, said that California had twice the seniority of any other delegation.

"One of the reasons that California accumulated so much clout was that, in 2002, the Legislature passed an incumbent protection act that rendered none of the districts competitive," Wasserman said, referring to the last redistricting map. "It helped California accumulate tremendous clout."

Wasserman predicted that more incumbents would retire in the weeks ahead, chased out by the fear of running in districts filled with unfamiliar voters. There have been six retirements so far. On the Republican side, beside Lewis, there are Elton Gallegly and Wally Herger, both of whom took office in 1987. On the Democratic side, the retirement roster includes Lynn Woolsey (1993), Dennis A. Cardoza (2003) and Bob Filner (1993). In addition, Rep. David Dreier, a Republican who has been in Congress since 1981, is said by colleagues to be likely to step aside.

This is not to say that California is heading for complete congressional Siberia. Pelosi will no doubt be in Washington next year. Rep. Kevin McCarthy, a Republican from Bakersfield, is the majority whip. And Darrell E. Issa, a Republican, is chairman of the House Oversight and Governmental Reform Committee and, as such, is arguably one of the more powerful members of Congress.

Pelosi, however, said that seniority was overrated.

"Yes, we have new people coming in," she said, in an interview. "We have people who won’t be coming back. But in terms of the influence of this state, we have plenty of people here who have standing on issues."

McCarthy said the changes were both inevitable, given the age of members who are going to retire sooner than later, and would ultimately be good for the state.

"You are losing that older skill set," he said. "But you are putting in someone who is younger; maybe they work a little harder."

Still, it is hard not to argue that there will be some drawbacks. Dreier is chairman of the Rules Committee, Berman is the ranking Democrat on the Foreign Affairs Committee and Woolsey is co-chair of the Progressive Caucus.

"Jerry Lewis is a real loss to California," Waxman said. "As chairman of the Appropriations Committee, he always looked out for the state — and not just his district. And without David Dreier there, it’s going to be a long time until we have a chance to have a California chairman of the Rules Committee on either side of the aisle."

For her part, Pelosi said she welcomed the independent redistricting process in California, noting that it would almost certainly increase the size of the state’s Democratic delegation next year.

"I’ll tell you this: If the Legislature had been in charge of redistricting California, they probably would have never given us four seats, because they would have been accused of being political," she said. "So this independent commission turned out better for us."

© 2012 The New York Times Company