“After an illness or an injury, many Americans who enroll in these G.O.P. junk health coverage plans will end up being hit by crushing medical bills, finding that they have been paying for coverage that doesn’t cover much at all,” said Representative Nancy Pelosi of California, the House Democratic leader.

Under the current rule, issued in late 2016 by the Obama administration, short-term insurance cannot last for more than three months, as it was meant to be a stopgap. Under the new rule, the limit would be 364 days, and insurers would be allowed, but not required, to extend policies. The maximum duration, including any extensions, would be 36 months.

The new options will help people struggling to afford coverage under the 2010 law, said Alex M. Azar II, the secretary of health and human services. “These plans aren’t for everyone,” he said, “but they can provide a much more affordable option for millions of the forgotten men and women left out by the current system.”

The new rule is presented as a redefinition of “short-term, limited-duration insurance.” But it stretches the common understanding of those terms, and some of the new policies could be an attractive option for healthier consumers who now pay high prices for major medical coverage and are willing to take more risk in return for lower prices.

“Short-term is getting longer!” one insurance marketing company advised clients this week.

Randy Pate, a senior official at the Centers for Medicare and Medicaid Services, said the Trump administration expects 600,000 people to buy the new insurance policies next year, with enrollment increasing to 1.6 million by 2022.