WASHINGTON — Obama administration officials are predicting that the number of Americans with private health insurance through the Affordable Care Act will grow by about 1 million in 2017, an upbeat final forecast for the president’s signature domestic achievement before he leaves office.

Health and Human Services Secretary Sylvia Mathews Burwell announced Wednesday that an estimated 11.4 million people, on average, will have health plans bought through the law’s insurance marketplaces during the coming year.

The administration’s optimism contrasts with recent estimates by outside analysts, some of whom have suggested that enrollment growth will slow or even reverse in 2017 because some insurers are withdrawing from the ACA exchanges and the prices charged by the remaining companies are soaring in many parts of the country.

“Ultimately, the marketplace is strong – and will continue to be strong – because it is offering a product that people want and need,” Burwell said in remarks at HHS headquarters.

HHS also announced Wednesday that the administration is expecting 13.8 million people to choose or automatically be re-enrolled in ACA health plans during the coming enrollment season – 1.1 million more than during this past year. As of mid-2016, the number who had insurance in place was 10.5 million. Nearly 85 percent were receiving federal subsidies.

The annual HHS forecast comes about two weeks before the marketplaces open on Nov. 1 for a fourth year’s enrollmentperiod. Seven days later will be the presidential election, and the ACA has been a bright dividing line between the main candidates’ policy views.

Branding it “terrible,” Republican Donald Trump has pledged to “completely repeal” the six-year-old law; like many others of his party, he points out what he says are myriad signs that it is failing.

Democrat Hillary Clinton is an avowed ACA supporter, though she wants to expand federal assistance to make health plan options more affordable and allow the marketplaces to include a government insurance alternative.

Against that partisan backdrop, the attractiveness of the insurance marketplaces carries an outsized significance this fall, even though they reach only a fraction of Americans with private insurance – those who cannot get affordable health benefits from a job.

“It’s part substantive in that enrollment has to increase in order to stabilize the market,” said Larry Levitt, senior vice president at Kaiser Family Foundation. “But it’s also kind of a game of perceptions. If enrollment decreased, it would create this perception that the law was in trouble.”

The upcoming sign-up period represents asignificant pivot point for the law. Despite the GOP’s persistent attempts to discredit and dismantle it, the ACA has survived two Supreme Court challenges and the disastrous 2013 debut of HealthCare.gov, when computer defects blocked many consumers’ ability to get coverage through the new federal health exchange. Along the way, the law has helped to bring about a large decrease in the ranks of the unisured.

The biggest problem now facing the federal marketplace and similar ones run by the District of Columbia and 16 other states is a matter of who has signed up: too many Americans with illnesses that consume a lot of medical care, and too few young adults and other healthy people to help balance out insurers’ costs.

That dynamic lies behind the defection of major insurers, including United HealthCare and Aetna Health, from some marketplaces, leaving nearly 20 percent of the people who now have ACA coverage with only a single insurer available for next year.

Goldman Sachs analysts predicted last month that, with insurers leaving and prices changing, the number of people in ACA health plans is likely to drop by roughly 20 percent next year. Last week, Standard & Poors analysts issued a forecast entitled, “Slowing Down: ACA Marketplace Growth May Halt in 2017.” It predicts that enrollment will range between 10.2 million and 11.6 million.

Congressional budget analysts also provide forecasts. Their most recent anticipated 11 million people with marketplace plans over the course of this year and 15 million during 2017. Next year, they estimate, one in five enrollees will not qualify for subsidies.

In 2015, CBO predicted that this year would usher in the biggest surge of enrollment in ACA coverage, with 20 million Americans in such health plans – twice what HHS had projected. The budget analysts’ most recent projection anticipates 19 million a decade from now – several million fewer than they had expected after the law was passed in 2010. Today’s numbers are lower in part because fewer employers than expected have stopped offering health benefits.

In Wednesday’s forecast, HHS officials did not include the central figure of its forecasts in the past: how many people it expects to have ACA health plans by the end of the coming year. Instead, it anticipated the average enrollment during 2017.

Katie Martin, HHS’s assistant secretary for planning and evaluation, insisted Wednesday that “we are not moving the goalpost. We are using what we believe is a more meaningful metric.”

The law requires most Americans to have health coverage. The penalty for remaining uninsured now stands at $695 per adult or 2.5 percent of income, whichever is higher.