Trump in October abruptly cut off federal payments that reimburse insurers for reducing out-of-pocket costs for lower-income Americans.
The so-called cost sharing reduction payments are the subject of a legal battle, with congressional Republicans arguing they cannot be paid without an appropriation by Congress.
But the Trump administration's decision to suddenly back the legal challenge after nearly a year of making the payments drew widespread criticism, especially coming so close to the 2018 open enrollment period.
And although a bipartisan group of senators has proposed legislation to fund the payments and stabilize markets, Trump has signaled he may not sign it.
Uncertainty over the payments has prompted many insurers to significantly raise their 2018 rates, in some cases by as much as 30 percent, according to a recent Kaiser Family Foundation analysis.
Many consumers will be protected from these rate hikes, in part because of premium subsidies provided through the health care law.
The law offers aid to consumers making between 100 percent and 400 percent of the federal poverty line, or between $12,060 and $48,240 a year.
Many insurers also concentrated the rate hikes in certain plans. Because of the complex way the law calculates subsidies, that means that some consumers could actually see their rates decline next year.
But Americans who make too much to qualify for aid -- many of whom have already experienced big premium hikes in recent years -- may see very large rate hikes again in 2018.
That could force some to drop coverage, weakening the markets further in some parts of the country.