Enrollment for 2018 ACA coverage is falling behind the pace of previous years. (Courtesy of Avalere Health/Courtesy of Avalere Health)

The number of Americans getting Affordable Care Act health plans for the coming year accelerated last week in states relying on the federal insurance exchange, bringing the total to 3.6 million sign-ups with less than two weeks left in an abbreviated enrollment season.

The latest federal snapshot, coming amid fresh political turbulence over the future of insurance marketplaces created under the law, is slightly ahead of the first five weeks' pace last fall. But compared with data from two-thirds of the way through the longer enrollment seasons of past years, the number of consumers who have chosen health plans is lagging far behind.

The 3.6 million figure is half of the total at the comparable point in the sign-up period for 2017 coverage, according to an analysis by the Washington-based consulting firm Avalere Health. To reach the 9.2 million enrollees that states relying on the federal marketplace had by the final deadline, a huge surge of people would need to take action by the time the season concludes on Dec. 15 or be automatically re-enrolled just afterward.

Last year, about 1.6 million people had their coverage automatically renewed in the 39 states using the federal HealthCare.gov website. If that number holds steady this year, it would mean about 4 million consumers would have to sign up by the end of next week or the country could see a sharp drop in how many Americans are insured through the ACA.

Since the health-care law's marketplaces opened in the fall 0f 2013, no two-week span has attracted that many enrollees.

The final gap may not be as wide as it appears now, because each enrollment season has had an upsurge as deadlines approach, said Larry Levitt, senior vice president of the Kaiser Family Foundation. Still, he said, "It seems a near-certainty that enrollment will end up well below last year."

The last stages of this year's six-week sign-up period coincides with yet another barrage against the law by congressional Republicans — on top of steps by the Trump administration to undercut the marketplaces. After failing this summer and fall to pass various GOP bills that would have dismantled significant parts of the ACA, the Senate has included in a massive tax bill it passed late last week a plan to end enforcement of the law's requirement that most Americans carry health insurance.

This provision will now be part of negotiations with House Republicans to try to resolve differences between the two chambers' versions of tax legislation.

In the weeks leading to the sign-up season, public opinion research and the ground-level experiences of enrollment helpers around the country suggested widespread public confusion about whether the marketplaces would still exist. It is unclear how enrollment might be affected by the prospect of the tax penalties for flouting the mandate no longer being enforced.

Democrats and other allies of the sprawling health-care law contend that the mandate is a crucial ingredient to keep insurers selling ACA health plans and to prevent monthly premiums from spiking further.

Since the marketplaces opened, though, there has been scant study of how much difference the mandate has made in prompting previously uninsured people to become covered. Research in Massachusetts, which adopted a health-care law in 2006 with many similarities to the ACA, found that its mandate appeared to motivate some residents to get a health plan.

Other indications are that, nationally, the bigger influence may be the federal subsidies available to more than 80 percent of people eligible for ACA coverage.

But overall, "it is hard to do reliable projections on what is motivating individual behavior," said Caroline Pearson, senior vice president at Avalere.

The nonpartisan Congressional Budget Office has estimated that ending enforcement of the individual mandate would translate into 13 million more people without coverage over a decade, but its analysts also have said they are in the midst of rethinking the basis of their calculation.

Though President Trump and others in the GOP now passionately oppose the mandate, the idea of some kind of insurance requirement came from conservatives during the early-1990s debate over President Bill Clinton's ultimately unsuccessful health-care overhaul plan. During the 2008 Democratic presidential primary campaign, the idea split two leading candidates, with Hillary Clinton espousing it and Barack Obama contending a mandate was not essential to broaden health insurance — a position he reversed once becoming president.

The latest enrollment snapshot, issued by the Centers for Medicare and Medicaid Services, is for the fifth week of sign-up, through this past Sunday. The 823,000 people who enrolled in coverage was about 300,000 more than for the previous week. It was also 13,000 more than the same week a year ago, while the cumulative tally of 3.6 million is 700,000 ahead.

The figures exclude enrollment in 11 states that operate their own insurance marketplaces under the ACA. People choosing health plans actually become insured once they begin to pay the premiums.