President Obama and his advisers hope the overhaul will do two things. The first is to extend coverage to tens of millions of Americans who today lack health insurance. The second is to hold the line on rising health care costs.

“Over time, success will depend on what happens to the cost curve,” Professor Cutler told me. “If we don’t bend the cost curve, everything will fail. The government won’t be able to afford it. Nobody will be able to afford it.”

In theory, the overhaul could meet both goals. Millions of new Americans armed with a subsidy and shopping among plans would bring consumer choice to bear, finally, on the health care industry. Insurers would compete to create policies that offered the most value for money, pressuring hospitals and doctors on behalf of all of us.

Yet despite the care the administration took in establishing incentives and safeguards, even some of Obamacare’s most committed backers are wondering whether the experiment will work as advertised — or, like Harvard’s P.P.O., go off the rails along the way.

Adverse selection is perhaps the direst threat. For Obamacare to work, millions of healthy, young, uninsured Americans must join a health plan to counterbalance the sicker millions who are most likely to buy insurance. Otherwise, health plans on the exchanges will have to raise premiums to shoulder the higher costs.

Selection will also take other forms. Healthier Americans will probably flock to cheaper bronze plans. And insurers will vie to enroll the healthy. In some states, big insurers have chosen not to participate in exchanges to avoid their strictures. On the outside, they could still sell cheap plans to skim off the healthy and avoid a rule that insurers on the exchanges must also offer more generous silver and gold plans.

Adverse selection is not the only risk.

A few studies have found that more competition among health insurers leads to lower hospital fees on average and that premiums rise when insurer competition diminishes. But researchers have also found that top hospitals — which any decent plan must have on its network — increase their fees when more health plans compete for their business.