Martha Lucia, left, consults Rudy Figueroa, a health-insurance agent, as she selects an insurance plan available under the Affordable Care Act at an event in the Mall of the Americas in Miami last year. (Joe Raedle/Getty Images)

IT HAS been a summer of bad news for the Affordable Care Act, but last week brought some numbers that should put worries about the law into perspective. The Census Bureau announced Tuesday that the proportion of people in the United States who lack health-care coverage continued to plunge last year — to only 9.1 percent. This figure is even better than it looks for Obamacare, because it factors in uninsured undocumented immigrants, of which there are perhaps several million, who are not eligible for the law’s programs.

But the overall number could be cut much lower, and quickly, if Obamacare were working as it was meant to. We are not referring to the recent, much-discussed exit of some major health insurers from the marketplaces the law created. We are talking about Obamacare’s expansion of Medicaid, the state-federal health plan for the poor and near-poor. The Supreme Court in 2012 made the expansion optional for states, and a large chunk, including Virginia, have refused. The Census Bureau found that the uninsured rate was 7.2 percent in expansion states last year and 12.3 percent in non-expansion states. Five states have expanded since, but that still leaves 19, representing 4 million to 5 million people who would otherwise get coverage, irrationally holding out.

Why irrationally? In their effort to hobble Obamacare, state Republican leaders have left huge amounts of federal money on the table. The federal government has offered to pay nearly the whole cost of the expansion, forever. Though states must pitch in a bit, they get a much lower uninsured rate, lower uncompensated care costs and other savings in return. The Urban Institute found last month that the 19 holdout states would get an average of $7.48 from the federal government for every dollar they spent on Medicaid expansion. Even those costs, meanwhile, would likely be further offset by savings elsewhere. States that have already expanded, in fact, have generally seen net revenue gains.

Other expansion critics argue that Medicaid does not help its beneficiaries enough to be worth the costs. But a recent National Bureau of Economic Research working paper found that Medicaid expansion has significantly reduced medical debt. People who got Medicaid coverage under Obamacare saw reduced medical collection balances of $600 to $1,000. Those who required a hospital stay or an emergency room visit saw a medical collection balance cut of $1,400 to $2,300.

The Affordable Care Act still has a long way to go. Its marketplaces need to stabilize further and enrollments must rise, which will be increasingly hard as the remaining uninsured are more and more difficult to reach. Congress and the White House have the primary responsibility for making the policy work. But it is also long past time for state GOP leaders to stop hurting their constituents in an effort to sabotage the law.