Millions of Americans face the prospect of significantly higher health insurance costs as a legislative package aimed at ending the longest government shutdown in U.S. history fails to address expiring Affordable Care Act tax credits. The bipartisan deal, which advanced Sunday with support from Senate Republicans and several Democrats, only promises a December vote on the enhanced premium tax credits set to expire at year’s end.

The absence of a clear resolution creates mounting uncertainty for healthcare consumers nationwide. House Speaker Mike Johnson has not committed to holding a corresponding vote in the Republican-controlled House, diminishing hopes for an extension of these critical subsidies that have made private health insurance more affordable for millions.

Democratic lawmakers have proposed one or two-year stopgap measures to preserve the subsidies, but Republican leadership has shown little interest in these compromises. Meanwhile, some Republican lawmakers and former President Donald Trump have advocated letting the subsidies expire entirely, suggesting alternative approaches such as federal flexible spending accounts for eligible Americans.

If Congress fails to act, the enhanced premium tax credits that have supported ACA health insurance plans for four years will disappear, causing dramatic cost increases for consumers. According to analysis by KFF, a health care research nonprofit, subsidized enrollees would face more than doubled premium costs on average.

“Hardest hit will be a small number of higher earners who will have to pay a lot more without the extra subsidies and a large number of lower earners who’ll have to pay a small amount more,” explained Cynthia Cox, vice president and director of KFF’s ACA program.

The impact would likely cascade through the healthcare system. As younger, healthier individuals abandon increasingly expensive coverage, insurers would likely raise premiums further for the remaining pool of older, sicker enrollees. A rise in the uninsured population could also shift costs to hospitals and government programs when people cannot pay for emergency care.

The effects are already being felt during the current open enrollment period that began November 1. Christine Meehan, a 51-year-old Pennsylvania hair stylist who depends on marketplace health insurance, told The Associated Press her monthly premium is set to increase by about $100 next year, jumping from $160 to approximately $260. Her annual deductible will also rise.

Despite Congressional inaction thus far, lawmakers could still extend the tax credits before expiration or reinstate them afterward. Such a move would align with public sentiment, as a recent KFF poll conducted in late October through early November found that approximately three-quarters of U.S. adults—including about half of Republicans—support extending the expiring tax credits.

If lawmakers do act to preserve the subsidies, federal and state marketplaces would need to adjust their available plans. Insurance experts indicate this adjustment is possible but could take weeks and grows more complicated with each passing day.

The delay might already have consequences, as some potential enrollees who have witnessed the projected price increases may have abandoned plans to obtain coverage. “They just might react and say this is too expensive for me, I can’t afford this anymore,” Cox noted. “There’s no guarantee that you can get that person to come back and shop again.”

Alternative approaches to healthcare affordability remain under discussion. Trump recently proposed redirecting funds that would have extended subsidies “directly to the people,” criticizing the ACA and suggesting Americans should have flexibility to purchase various types of care.

Senator Bill Cassidy, who chairs the Senate health committee, has proposed creating “pre-funded federal flexible spending accounts” for eligible Americans to use for health expenses ranging from dental care to prescription drugs. Similarly, Senator Rick Scott announced he’s developing legislation to implement Trump’s suggestion by establishing health savings account-like funds.

These alternative proposals remain conceptual and far from implementation. Their potential impact on healthcare affordability remains unclear, especially compared to the established benefits of the current subsidy structure that millions of Americans have come to rely upon.

As the shutdown-ending legislation advances without addressing this critical healthcare issue, millions of Americans face a holiday season of uncertainty about their healthcare costs for the coming year.