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 6 members retiring and strong contests in 10 other 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 in the nation, is sinking in with every new retirement announcement. 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 out 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.