About 700,000 adults between ages 26 and 49 will be eligible as of Jan. 1.
California will welcome the new year by becoming the first state to offer health insurance for all undocumented immigrants.
Starting Jan. 1, all undocumented immigrants, regardless of age, will qualify for Medi-Cal, California's version of the federal Medicaid program for people with low incomes.
Previously, undocumented immigrants were not qualified to receive comprehensive health insurance but were allowed to receive emergency and pregnancy-related services under Medi-Cal as long as they met eligibility requirements, including income limits and California residency in 2014.
In 2015, undocumented children were able to join Medi-Cal under a bill signed by then-Gov. Jerry Brown. In 2019, Gov. Gavin Newsom signed into law an expansion of full-scope Medi-Cal access for young adults ages 19 through 25, regardless of citizenship or immigration status. Access was then further expanded to allow older adults aged 50 and older to receive full benefits, also regardless of immigration status.
The final expansion going into effect Jan. 1 will make approximately 700,000 undocumented residents between ages 26 and 49 eligible for full coverage, according to California State Sen. María Elena Durazo.
"This historic investment speaks to California's commitment to health care as a human right," Durazo said in a statement in May.
"In California, we believe everyone deserves access to quality, affordable health care coverage - regardless of income or immigration status," Gov. Newsom's office said in response to an ABC News request for comment. "Through this expansion, we're making sure families and communities across California are healthier, stronger, and able to get the care they need when they need it."
California's plan to expand coverage has not been without its detractors. The California Senate Republican Caucus criticized the move in an analysis of the 2022-23 governor's budget.
"Medi-Cal is already strained by serving 14.6 million Californians - more than a third of the state's population. Adding 764,000 more individuals to the system will certainly exacerbate current provider access problems," the caucus wrote last year.
However, studies have shown that undocumented immigrants use fewer healthcare resources than do non-immigrants.
About 50% of undocumented immigrant adults in America report being uninsured, compared to just 8% of U.S.-born citizens, according to the health policy research nonprofit KFF, due to undocumented adults being more likely work jobs that don't provide health benefits, as well as facing eligibility restrictions for federal programs.
Additionally, undocumented immigrants who are eligible may face other barriers, including confusion about eligibility, language challenges and fear, KFF said.
This is despite low-income undocumented immigrants in California being about as likely as those currently enrolled in Medi-Cal to have at least one chronic condition, according to research conducted by the Public Policy Institute of California (PPIC), a nonprofit research institution.
Undocumented immigrants are also about as likely to receive preventive services, such as the flu shot, and older undocumented immigrants are about as likely to receive age-appropriate health services such as cancer screenings and the shingles vaccine, at rates similar to Medi-Cal patients, the PPIC found.
The California Health and Human Services Agency has committed $835.6 million in 2023-24 and $2.6 billion annually thereafter to expand full-scope Medi-Cal eligibility to all income-eligible adults, regardless of immigration status.
When California expanded health care coverage in 2019 to include undocumented young adults, then-President Donald Trump blasted the plan, calling it "very unfair to our citizens."
"If you look at what they're doing in California, how they're treating people, they don't treat their people as well as they treat illegal immigrants," Trump told reporters at the time. "So, at what point does it stop? It's crazy what they're doing. It's crazy. And it's mean, and it's very unfair to our citizens."