Dr. Swiger said the health care decision made him weep for his country. But to win his Senate campaign over Tim Kaine, a popular former governor, Mr. Allen needs to reach less partisan voters like Joshua Schwanborg, 43, the proprietor of China King restaurant. He was far more concerned about the rates charged by the Australian highway firm that runs the toll road to Washington.

Leaders in both parties acknowledge that the ruling has thrown a wrench into their campaigns for control of the House and the Senate. House Republicans have scheduled another vote next week to try to repeal the law, known as the Affordable Care Act. And they say they are ready to play offense on the reinvigorated health care debate.

But even as they highlight that mobilization, leaders of both parties say overemphasizing the health care issue could turn off weary swing voters who, they fear, just want to put the issue aside. Representative Greg Walden of Oregon, the deputy chairman of the National Republican Congressional Committee, put the health care law in fourth place on the campaign agenda — behind jobs and the economy, jobs and the economy, and jobs and the economy.

“I’d rather be on our side than theirs,” he said. “If Democrats want to go out there and talk about health care, have at it.”

The message is muddy for both parties, in part because neither is sure whether 2012 will turn solely on the economy or echo the dynamics of 2010, when the health care law was a driving force. Democrats know they cannot repeat their strategy of that election, when they simply avoided the subject of the historic health care law they had just passed, said Representative Steve Israel of New York, chairman of the Democratic Congressional Campaign Committee. That got them trounced.