In a report, the Department of Health and Human Services said Monday that there are around two million low-income, uninsured people in those 20 states who have a mental illness or substance abuse disorder.

Medicaid has long been a joint federal-state program that offers near-free care to the very poor. Under the health law, Washington pays almost all of the costs of insuring people who have slightly higher incomes.

Opponents of expansion argue that neither states nor the federal government can afford to further swell the program, and that a shortage of providers to treat the newly insured poses an additional challenge in trying to enroll more people in it.

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