It establishes a menu of preventive procedures, like colonoscopies, mammograms and immunizations, that must be covered without co-payments. And it allows consumers who join a new plan to keep their own doctors and to appeal insurance company reimbursement decisions to a third party.

The arrival of the long-awaited changes propelled President Obama, whose Democrats have struggled to exploit their signature achievement, into the backyard of Paul and Frances Brayshaw of Falls Church, Va., to explain his decision to pursue health care.

“The amount of vulnerability that was out there was horrendous,” Mr. Obama on Wednesday told a gathering of people chosen to illustrate the law’s new provisions. He said he concluded that “we’ve just got to give people some basic peace of mind.”

Mr. Obama also responded to Republican Congressional leaders who have campaigned on a threat to repeal the act. “I want them to look you in the eye,” he told his audience, and explain their opposition to a law that is projected to cover 32 million uninsured and reduce the deficit by $143 billion over 10 years.

The Republican strategy “makes sense in terms of politics and polls,” Mr. Obama said, an acknowledgment that the electorate is divided and that many swing districts are hostile. “It just doesn’t make sense in terms of actually making people’s lives better.”