A new Hawaii law sets a three-month limit on short-term plans and generally prohibits insurers from selling them to anyone who was eligible to buy comprehensive insurance through the Affordable Care Act marketplace in the prior year.

The Virginia legislature passed a bill authorizing insurers to sell short-term plans lasting up to 364 days, but Gov. Ralph S. Northam, a Democrat, vetoed it.

“People with minimal current health care needs are more likely to purchase these skimpy plans, leaving people who have more significant health care needs” to face higher premiums in the marketplace, Mr. Northam said.

The administration is standing by its push. William Brady, an associate deputy secretary at the federal Department of Health and Human Services, told state officials here that the new short-term plans could be “a great option for many Americans who were shut out of the insurance market by Obamacare’s high premiums.” He said that up to two million people were expected to enroll in the new plans at prices 50 percent to 80 percent lower than those of “Obamacare policies.”

But that discount comes at a price. Short-term plans do not have to cover prescription drugs, maternity care, mental health services or pre-existing conditions, which must be covered by Affordable Care Act plans.

“I have questions about the underlying value of these products,” Dave Jones, the California insurance commissioner, said after listening to an upbeat discussion of short-term plans by several insurance executives.

While two of the insurance executives were speaking, Mr. Jones inspected their websites on a laptop computer and said he could not find even basic information about the benefits and limitations of the policies. A bill passed by the California Senate and pending in the Assembly would bar the sale of short-term plans in the state.