Because the law’s long-term sustainability depends on millions more people signing up for health coverage in coming years, its supporters will have to reverse the erosion in coverage when the second sign-up period begins this fall.

“The challenge in the coming year will be signing up more people for coverage, especially reaching those who stayed on the sidelines this year,” said Larry Levitt, senior vice president at the nonprofit Kaiser Family Foundation, which has been closely tracking implementation of the law.

The law this year for the first time allowed Americans who don’t get health benefits at work to shop among plans on new marketplaces. Those making less than four times the federal poverty level — or about $94,000 for a family of four — qualify for subsidies.