California is poised to lose multiple congressional seats after the 2020 census for the first time in the state's history, thanks to an exodus of more than 200,000 people between 2018 and 2019, according to the Los Angeles Times. Top destinations include Texas, Arizona, Nevada, Oregon, Washington and Colorado.

"It’s got a lot to do with dispersion from California to the rest of the west," said senior Brookings fellow, William Frey. "Arizona, Texas and Colorado are all big destinations for California migrants, and they all are gaining seats."

About 203,000 people left California in that period, a result of the state’s shifting migration patterns and economic strains that are making it harder to afford living here. New York, Illinois, New Jersey, Massachusetts and Louisiana also saw large losses to other states. California’s potential loss in reapportionment, which will be determined by next year’s census count, would drop the state’s number of seats in the U.S. House of Representatives from 53 to 52, said William Frey, a senior fellow at the Brookings Institution. -LA Times

According to a relocation survey by Texas Realtors, 63,175 Californians moved to Texas in 2017, while almost 41,000 Texans moved to the Golden State. And while California may lose a seat in the House, Texas is likely to gain three seats after the 2020 census. Arizona, Colorado and Oregon may gain one seat apiece.

A state's population includes all residents - citizens and non-citizens, along with overseas federal employees and their dependents from whatever state they claim as home, according to the US Census Bureau.

That said, if illegal immigrants fail to participate or fail to give honest household figures, California could lose multiple seats according to state redistricting analyst Paul Mitchell,

"If, as many fear, non-citizen populations and the state’s heavily Latino population either fails to participate or participates without providing full household counts, then California could lose more than one seat," said Mitchell.

The House of Representatives, meanwhile, is limited to 435 members thanks to a 1929 federal law which lawmakers and the president could change if they wanted to.

Exactly where California would lose a seat in the House depends on which communities are larger or smaller compared to census numbers from 2010. The state’s Citizens Redistricting Commission, whose members will be selected in coming months, will hold public hearings in 2021 to determine how to redraw congressional maps. Paul Mitchell, one of the state’s leading analysts of the redistricting process, said that two places could dominate the discussion: the communities sitting at the intersection of Los Angeles, Orange and San Bernardino counties and the suburbs to the east of San Francisco. -LA Times

The most obvious political impact is that incumbent House members would need to run against each other or leave office. The Times notes that in 2012, "Rep. Brad Sherman (D-Northridge) defeated former Rep. Howard Berman in a bitter contest brought on by the new lines drawn in Los Angeles County."

Losing a House seat would be a "massive rewrite of the Central Valley congressional districts," said Mitchell.

California’s future numerical strength in Congress hinges in part on making sure that members of historically undercounted groups are included in the census count. In California, 72% of the population belongs to one of these groups, according to the Public Policy Institute of California. State census workers, community organizations and local politicians started outreach efforts as early as April to ensure an accurate tally in next year’s count. In addition to reapportionment, nearly $800 billion in federal tax dollars and political redistricting are at stake. -LA Times

Officials say Los Angeles county will be the hardest to accurately tally because of its high concentrations of renters, homeless people, and immigrants who may not participate due to language barriers or fear of being targeted by federal immigration authorities.

Meanwhile, the US growth rate has declined steadily over the past decade - with California leading the charge according to Frey, with about 400,000 people under the age of 18.

"This is a symptom of an aging population," Frey said, adding "and in states like California, an out-migration of younger families with children."