An optimistic Obama administration is predicting continued enrollment growth during the Affordable Care Act’s fourth sign-up season. (HealthCare.gov)

Obama administration officials are predicting that the number of Americans with private health insurance through the Affordable Care Act will increase 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 said Wednesday that an estimated 11.4 million people, on average, will have health plans from the law’s insurance exchanges during the coming year. The forecast is more optimistic than those of some outside analysts, who predict that enrollment growth will slow or even backslide.

The conflicting expectations arise from disagreements about the effect of upheaval in some exchanges as they approach their fourth year, with insurers defecting in parts of the country and the remaining ones sharply raising prices. Administration officials are playing down the impact.

Burwell used the annual enrollment prediction as a kind of paean to what she heralded as “the biggest step forward in a generation” — the administration’s work to usher in insurance that is better, more affordable and more broadly available. She acknowledged that future ACA proponents will need Congress as a partner to accomplish any “substantial changes” in the law, including a public insurance option to foster more market competition in places with few insurers.

The secretary contended that the law’s marketplace “is sustainable in terms of its size.” In remarks at HHS headquarters, she said that it is “strong — and will continue to be strong — because it is offering a product that people want and need.”

Officials said they expect 13.8 million people to choose or be re-enrolled automatically in ACA health plans during the upcoming enrollment period, which runs from Nov. 1 through Jan. 31. They predict that about 9.2 million will be current marketplace customers — an estimate that signifies a drop-off from the 10 million people whom HHS expects to have ACA coverage at the end of December.

The rest of the total will be people who have stayed uninsured, plus people who have individual policies outside the marketplaces.

The administration’s forecast comes three weeks before the presidential election, and the ACA has been a stark 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 supporter, though she wants to expand federal assistance to make health plans more affordable and to allow a public option — a position President Obama now embraces.

Against that partisan backdrop, the attractiveness of the insurance exchanges carries outsize significance this fall, even though they reach only a small fraction of Americans with private coverage: those who cannot get affordable health benefits from a job.

Despite the Republicans’ 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 hindered many consumers’ attempts 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 uninsured.

The upcoming sign-up period represents a significant pivot point for the ACA. The biggest threat now facing its federal marketplace and similar ones run by the District, Maryland and 15 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 withdrawal 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 Americans in ACA plans is likely to drop by roughly 20 percent next year.

And last week, Standard & Poor’s predicted between an 8 percent decrease and a 4 percent increase. Based on the HHS figures announced on Wednesday — that 10.5 million Americans had marketplace coverage in place as of mid-2016 — the low end of S&P’s projected range would translate into average enrollment next year of less than 10 million.

Both outside assessments suggest that HHS is underestimating how many Americans, with incomes too high for insurance subsidies, will drop marketplace coverage because of spiking ­prices.

Congressional budget analysts also provide forecasts. Their most recent estimate of ACA enrollment is 15 million people in 2017.

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

Kathryn Martin, acting assistant secretary for planning and evaluation, insisted that “we are not moving the goal post. 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.