Trump also roiled markets in October by directing federal agencies to look at ways to broaden the availability of cheaper, less comprehensive health plans which could leave only sicker, more expensive consumers behind in the marketplaces.
Trump's moves came on top of dramatic budget cuts by the Department of Health and Human Services for enrollment efforts this year, slashing advertising by 90 percent and reducing federal funding for organizations that help people sign up by more than 40 percent.
At the same time, this year's enrollment period -- which runs from Nov. 1 to Dec. 15 -- will be half as long as prior enrollment periods for residents of the 39 states that use the federal HealthCare.gov marketplace.
Most state-run marketplaces, including California's and Connecticut's, plan to keep their enrollment periods open into January.
"This is like the invisible open enrollment," said Peter Lee, executive director of Covered California, the state's insurance marketplace. "That's a prescription for low enrollment and bad risk."
The marketplaces, which began offering health coverage in 2014, depend on attracting enough younger, healthier enrollees to offset the costs of older, sicker consumers. That helps keep premiums in check.
Getting that mix right was challenging before this year, as the Obama administration labored to convince younger consumers to sign up for coverage.
Some states are again planning robust new outreach efforts for 2018.
And some health insurers will increase marketing efforts this year, said Jeanette Thornton, senior vice president at America's Health Insurance Plans, the industry's Washington-based lobbying arm.
But the Trump administration has no major plan to promote enrollment this year and has done little to coordinate with grass-roots groups that help people sign up for health plans, according to several leading groups.