Insurers will again be able to sell short-term health insurance good for up to 12 months under a proposed rule released Tuesday by the Trump administration that could further roil the marketplace.

"We want to open up affordable alternatives to unaffordable Affordable Care Act policies," said Health and Human Sservices Secretary Alex Azar. "This is one step in the direction of providing Americans health insurance options that are more affordable and more suitable to individual and family circumstances."

The new rule is expected to entice younger and healthier people from the general insurance pool by allowing a range of lower-cost options that don't include all the benefits required by the federal law — including plans that can reject people with preexisting medical conditions.

Also, according to the proposed rule, the plans would not be required to sell to everyone, so people with medical problems may not be able to get this coverage.

"It will undermine the individual market risk pool," said Kevin Lucia, research professor and project director at Georgetown University's Health Policy Institute.

Over time, those remaining in ACA plans will increasingly be those who qualify for premium tax credit subsidies and the sick, who can't get an alternative like a short-term plan, predicts Lucia and other experts. That, in turn, would drive up ACA premiums further.

"If consumers think Obamacare premiums are high today, wait until people flood into these short-term and association health plans," said industry consultant Robert Laszewski. "The Trump administration will bring rates down substantially for healthy people, but woe unto those who get a condition and have to go back into Obamacare."

Supporters said the rules are needed because the ACA plans have already become too costly for people who don't receive a government subsidy to help them purchase the coverage. "The current system is failing too many," said Seema Verma, who now heads the Centers for Medicare & Medicaid Services, which oversees the marketplaces.

And, many supporters don't think the change is as significant as skeptics fear.

"It simply reverts back to where the short-term plan rules were prior to Obama limiting those plans," said Christopher Condeluci, a benefits attorney who also served as tax counsel to the U.S. Senate Finance Committee.

But in their call with reporters, CMS officials noted that the proposed rule is seeking comment on guaranteeing the renewability of these short-term plans. That would be different than prior practice.

The comment period for the proposed rule runs for 60 days.

The directive follows an executive order issued in October to roll back restrictions put in place during the Obama administration that limited these plans to three months. The rule comes on the heels of Congress' approval of tax legislation that in 2019 will end the penalty for people who opt not to carry insurance coverage.

The administration also issued separate regulations Jan. 4 that would make it easier to form "association health plans," which are offered to small businesses through membership organizations.

Together, the proposed regulations and the elimination of the so-called individual mandate by Congress could further undermine the Affordable Care Act marketplace, critics say.

Verma told reporters Tuesday that federal officials believe that between 100,000 and 200,000 healthy people now buying insurance through those federal exchanges would switch to the short-term plans, as well as other people who are now uninsured.

Short-term plans had been designed as temporary coverage, lasting for a few months while, for instance, a worker is between jobs and employer-sponsored insurance. They provide some protection to those who enroll, generally paying a percentage of hospital and doctor bills after the policyholder meets a deductible.

They are generally less expensive than ACA plans, because they cover less. For example, they set annual and lifetime caps on benefits, few cover prescription drugs, and most exclude coverage for maternity care, preventive care, mental health services or substance abuse treatment.

Most require applicants to pass a medical questionnaire — and they can also exclude coverage for preexisting medical conditions.

Extending short-term plans to a full year could be a benefit to consumers because they must pass the health questionnaire only once. Still, if a consumer develops a health condition during the contract's term, that person would likely be rejected if he or she tried to renew.

Both supporters and critics of short-term plans say consumers who do develop health problems could then sign up for an ACA plan during the next open enrollment because the ACA bars insurers from rejecting people with preexisting conditions.

"We're going to have two different markets, a Wild West frontier called short-term medical … and a high-risk pool called Obamacare," said Laszewski.

KHN senior correspondent Phil Galewitz contributed to this article.