The Affordable Care Act’s insurance marketplaces open for a sixth year Thursday with more stable health plan choices and rates, plus significant tests of the effects of recent Republican moves to undercut parts of the law.

During the enrollment season — which lasts six weeks, half as long as it used to be — consumers may buy the health coverage created under the ACA. This is the first enrollment since Congress removed the law’s penalty for people who fail to carry health insurance. With that federal fine scheduled to vanish in January, this year’s marketplaces will furnish evidence for a long-simmering debate: How much of the nation’s gains lately in health coverage have happened because of the law’s insurance mandate, and will coverage tumble without it?

This enrollment season will also be the first since the Trump administration has been taking steps to circumvent the ACA’s insurance requirements, making it easier for individuals to buy two inexpensive types of insurance that cover less care and lack certain popular consumer protections.

Administration officials tout this alternative coverage, short-term health plans and “association health plans,” as expanding affordable options, while critics predict it will confuse consumers, leading some to buy less coverage than they need. In a major switch, federal health officials have just said they are open to states using the ACA’s subsidies to help people afford insurance premiums for these health plans, but states have not had time to get permission for this enrollment period.

In fostering such new policies, the administration’s senior health officials have been walking a balance beam in the lead-up to the open enrollment: They are claiming credit for causing the marketplaces to be less turbulent than in the past few years, while continuing to argue for an end to the sweeping 2010 health-care law that was President Barack Obama’s prime domestic achievement.

“We have been working tirelessly to fulfill the promises we made to stabilize the market,” Seema Verma, administrator of the federal Centers for Medicare and Medicaid Services, which oversees much of the ACA, said in mid-October, as her agency released an annual compendium of insurance rates in the ACA’s marketplaces. It shows that, for the first time, the average premiums for the most popular level of coverage is dipping slightly for 2019 — by 1.5 percent, with wide variation from state to state.

How much credit the administration deserves for ACA premiums leveling off is a matter of dispute. An analysis by the Kaiser Family Foundation, a health policy organization, found that rates for the most popular level of ACA health plan are, on average, 16 percent higher than they would have been without the changes by the administration and Congress.

Unlike during the Obama era, when high-wattage promotion of the ACA marketplaces was an annual ritual, federal health officials for a second year have not made a public prediction of how many people they expect to sign up. For 2018 coverage, enrollment dipped slightly, to nearly 12 million.

The marketplaces were intended to help individuals and families who cannot get an affordable health plan through a job. In the states relying on the federal marketplace, consumers sign up mainly through the website and federal call centers.

The 11 other states and the District run their own exchanges; several of those are planning a longer enrollment period, through December or into January.

A year ago, President Trump unilaterally ended one of two major subsidies built into the law — payments to insurers to cushion them from discounts they must give lower-income customers on out-of-pocket costs. The change prompted widespread predictions that insurers, many of whom had already been defecting from the marketplaces, would flee.

For the most part, though, they stuck around, having already planned on big rate increases out of fear that Trump would take the action that he did. In signs that the marketplace’s jitters have subsided in the past year, fewer areas of the country have only one insurer in their marketplaces. In 15 states, insurance companies are returning or entering for the first time, Kaiser data shows.

One big change this year is the amount of neutral guidance insurance-shoppers will be able to find. The ACA created grants to grass-roots organizations known as navigators in the states using the federal exchange, and the Trump administration has, over the past two years, curtailed money for such groups from about $63 million in 2016 to $10 million this year.

CMS’s Verma has argued that navigators are less needed now that Americans have had time to become familiar with ACA marketplaces. She and other administration officials have been trying to foster a larger role for private insurance agents and brokers. Earlier in October, CMS announced a hack in the online system used by those agents and brokers. The system was temporarily closed, reopening last week.

For consumers accustomed to getting help from navigators, the decrease will be stark. Of 34 states that received navigator grants two years ago, all but one have lost at least three-fourths of their funding. Three — Iowa, Montana and New Hampshire — have no navigators left.

Fred Ammons, chief executive of a nonprofit group that oversees Insure Georgia, the only navigator group available statewide since 2014, said his group did not get a grant this year, leaving only a smaller group that in the past focused on helping refugees around Atlanta. Some insurance agents, Ammons said, are charging $200 enrollment fees, so several staff members of his group have gotten certified as the only nonprofit agents in the state.

Still, 10 or so Insure Georgia agents cannot do the work more than 40 navigators used to do. “I believe there is a little bit of a false narrivative it is going to be okay,” Ammons said.