Health and Human Services Secretary Sylvia Burwell said she expects 13.8 million people to sign up.
This is shaping up to be the most difficult sign-up season since HealthCare.Gov launched in 2013 and the computer system froze up. But technology isn't the issue now.
Premiums are going up sharply in many parts of the country, and some major insurers have exited the program, leaving consumers with fewer choices next year.
The administration says taxpayer-provided subsidies will cushion most of the impact of premium increases that are expected to be well into the double digits in many states.
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It's up to the consumer whether or not to accept the match or keep shopping.
Going into its fourth sign-up season, President Barack Obama's health care law has yet to achieve stability.
Enrollment has been lower than initially projected, insurers say patients turned out to be sicker-than-expected, and a complex internal system to help stabilize premiums did not work as intended, partly because of actions by congressional Republicans.
The law offers subsidized private insurance to people who do not have coverage through their jobs, along with a state option to expand Medicaid for low-income people.
More than 21 million people have gained coverage since the law passed in 2010.
The government said enrollment averaged more than 10 million people through the first half of 2016, and more than 8 in 10 were receiving financial help.
Nonetheless, the law remains politically divisive and Republicans are still vowing to completely repeal it.
The administration is hoping for a strong sign-up season to validate the president's signature program, and for a victory by Democrat Hillary Clinton in the presidential election to close out the long-running political saga.