"It's kind of insurance 101," said Jim Havens, who oversees individual market sales for Premera Blue Cross, a leading insurer in Washington state. Premera will be the only insurer on Alaska's marketplace next year.
"If you only have people who need care the most, insurance becomes really expensive really fast," Havens said. "A mandate is fundamentally so important to make insurance affordable."
Virtually every major physician group, hospital association and patient advocate group has called on congressional Republicans not to scrap the mandate.
"Repealing the individual mandate without otherwise increasing access to adequate, affordable health insurance is a step backwards for individuals and families," the American Heart Association, the American Lung Association, the March of Dimes, Consumers Union, the advocacy arm of the American Cancer Society and 11 other patient groups warned in a letter to Congress.
Access and affordability are already a challenge in many parts of the country, as insurers have struggled to adjust to new rules imposed by the health law, including requirements that health plans cover sick consumers and offer a basic set of health benefits.
And though many Americans who qualify for financial aid through the health care law have been cushioned from high premiums, consumers who earn too much to get insurance subsidies face eye-popping bills.
The health law provides premium assistance to people with annual incomes between 100 percent and 400 percent of the federal poverty level, or between $12,060 and $48,240.
There were signs that markets were stabilizing in many parts of the country until this year's GOP push to repeal the law, which included moves by the Trump administration to cut enrollment efforts and stop a separate monthly payment to insurers, used to lower the cost of deductibles and copayments for low-income consumers.
Those steps prompted many insurers to leave markets or dramatically raise premiums for 2018.
"All of that is driving the markets crazy," said Barbara Richardson, insurance commissioner in Nevada, where 11 of the state's 17 counties are among the 454 nationally with just one insurer and extremely high premiums. "Any time you shake up the market ... you are going to have a rise in prices."