Getting those marketplaces built and the states ready to accept a flood of applications are big challenges. Perhaps a bigger one is reaching all those eligible people and educating them about how they can benefit. Although public polling has shown consistent and strongly held views about the health law overall, public understanding of its individual provisions has always been low. A recent survey conducted by the newly formed Enroll America found that 83 percent of people who will be eligible for Medicaid were unaware of their status, as were 78 percent of those eligible for tax credits.

"I can't think of any task that involved changes in America's health care system over the last half-century that is as big a challenge as reaching these tens of millions of people," said Ron Pollack, the executive director of Families USA, a health-reform advocacy group that has joined forces with the health industry and charitable groups to form Enroll America.

Enroll America is hoping to raise "tens of millions of dollars," according to Executive Director Rachel Klein, money that will be devoted to television and online advertising, as well as word-of-mouth education that can reach the uninsured in their communities. The group, which draws on resources from health insurers, hospitals, and religious and community groups, will conduct a nationwide push, with a greater focus in the states with the largest population of uninsured residents.

Enrollment levels will matter, because the number of Americans the law insures will largely determine whether the Affordable Care Act is considered a success or failure. But enrollment levels will also determine whether the law's basic apparatus will work. The Affordable Care Act is designed to bring young, healthy people into insurance markets, where they can help offset the costs of insuring the old and the sick. Populations that are already in constant contact with the health care system will be encouraged to sign up for insurance as soon as they can. But the healthy people the system will need are the ones who will be hardest to reach. Without their participation, the law may not succeed in making coverage affordable for the uninsured.

The federal government will be involved in the public outreach, and it is developing a strategic plan, built on its enrollment efforts after the enactment of the Children's Health Insurance Program in 1997 and Medicare Part D in 2003. Political considerations, however, might limit the government's efforts. House Republicans have already called for an investigation into the decision to devote government funds to the promotion of the health care law. To critics, such advertising smacks of political advocacy.

The role of states may be more variable. In Massachusetts, the only state to have rolled out a similar system of expanded coverage so far, politicians and the business community pulled together in support of the law, and a clever advertising campaign, linked to the Boston Red Sox, told young, healthy, uninsured residents that they should buy insurance. More than 95 percent of the state's residents are now insured. Several people involved in the effort say that the marketing campaign, and the uniformity of the public message about the law, were key to its success.