Despite their failure to repeal the Affordable Care Act this year, congressional Republicans managed to pass a measure that could spell the end of the program as we know it: a repeal of the individual mandate provision.

The individual mandate, which requires most Americans to have health insurance or face a hefty tax penalty, has never been popular. But it’s necessary for a functioning health care marketplace. To keep premiums affordable, insurance companies need everyone, especially younger and healthier people, to be in the pool.

So the mandate repeal — which congressional Republicans shoehorned into the major tax overhaul they passed last week — could have a huge impact on the ACA’s future.

The Congressional Budget Office estimates that repealing the tax penalties for Americans who don’t have insurance will increase premiums by 10 percent and leave 4 million more people without coverage in 2019 — and 13 million more by 2027.

California has a lot at stake. After moving quickly to implement the Affordable Care Act and launching a relatively successful state-run health care marketplace in Covered California, the state has seen its uninsured rate drop to a record 7 percent.

Keeping those gains won’t be easy.

“The penalty going away is significant,” said Peter Lee, Covered California’s executive director. “The penalty is not, however, the glue that holds the Affordable Care Act together.”

California still has options for maintaining a healthy state insurance marketplace, even without the federal mandate.

Choosing the right one will be a crucial decision for state lawmakers.

The simplest option would be for California to pass its own individual mandate. Massachusetts has one, passed in 2007 as part of its health care reform under then-Gov. Mitt Romney.

But a state-level mandate is unlikely. It would require a two-thirds vote in the Legislature. That’s unlikely at the best of times and even less likely now, when several Democratic lawmakers have resigned over sexual harassment allegations and many others are receiving blowback over the new gas tax.

These fiscal realities complicate the odds for another clear option as well. In response to potentially higher premiums, California could choose to expand its premium subsidies.

“The state could expand subsidies to people who are making a little too much for them now,” said Laurel Lucia, director of the UC Berkeley Labor Center’s health care program. Lucia also suggested helping older people who don’t currently qualify for subsidies — but whose premiums are much higher than younger ones.

But any expansion of the state’s premium subsidies would be a direct hit to the state’s general fund, with no guarantee of assistance from Washington.

That’s why California’s best bet may be a state re-insurance program.

A reinsurance program means the state would offer insurance to insurance companies on high-cost claims. By limiting company liability on the highest-cost claims, the insurers can offer everyone lower premiums.

It costs state money to create these programs, which were recently approved in Alaska, Minnesota and Oregon. But state reinsurance saves money for the federal government, by lowering the overall cost of premium subsidies. In Alaska, Minnesota and Oregon, the federal government has agreed to return its savings to the states — thereby offsetting the cost of the re-insurance program.

Whatever option California lawmakers pursue, they must also make sure to properly fund marketing and outreach for Covered California. It sounds almost too simple, but it’s true: People won’t participate in the marketplace if they don’t know what’s available.

“The reason people are buying coverage is not the penalty, it’s because they see the deals they’re getting,” Lee said. “If people can get affordable coverage, they want it.”

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