WASHINGTON — Their positions are clear.

President Barack Obama ardently defends his federal health care overhaul. Republican challenger Mitt Romney adamantly opposes it. But this coming week, when the Supreme Court rules on the constitutionality of the law, both sides will be scrambling for political gain no matter the outcome.

If the court upholds the law, Obama will get vindication for his signature legislative accomplishment. Romney will have a concrete target for his pledge to repeal it.

If the court rules against part or all of the law, Obama could blame Romney, congressional Republicans and a conservative-leaning court for denying health benefits to millions of people in the United States. Romney could claim victory for his assertion that the government overreached.

Striking down all or part of the law less than five months from Nov. 6 election also could mean much political uncertainty for both campaigns. That could force them to reshape long-held strategies and try to satisfy voter demands for Washington to start anew on fixing a broken health care system.

Just one-third of those questioned back the law, according to an Associated Press-GfK poll this month. But there is overwhelming backing among both supporters and opponents for Congress and the president to find a new remedy if the high court strikes down the 2-year-old law.

Obama, in campaign appearances, promotes the law’s more popular elements, such as a provision allowing children to stay on their parents’ insurance up to age 26. The administration has also pumped out a steady stream of positive news, including a report this past week that nearly 13 million people would receive health insurance rebates averaging $151 per household.

Romney has emphasized his repeal pledge, a central pillar of his agenda that so far offers voters few other specifics. He released a television ad Friday stressing that the elimination of the health care law will be the top priority in his administration’s first 100 days.

“Day One, President Romney moves to repeal ‘Obamacare’ and attacks the deficit,” the narrator says in the ad set to run in Virginia, North Carolina and Iowa.

Obama aides have refused to discuss contingency plans to avoid creating the impression that they are preparing for the court to rule against them.

“We do believe it’s constitutional,” senior adviser David Plouffe said. “We obviously will be prepared for whatever decision the court renders.”

If the court rules in its favor, the administration would press forward with full implementation of the law and use the decision to ward off Republican criticism of the overhaul. The court could strike down the full law or just the most disputed element, that most individuals carry health insurance.

In either case, the president and his aides would try to portray the repeal as detrimental to the well-being of millions of people by highlighting popular aspects of the overhaul that would disappear, including guaranteed coverage for people with existing medical conditions.

In the event of a partial repeal, the administration would move ahead with those parts of the law that do survive and could still put coverage within reach of millions of uninsured people, lay new obligations on insurers and employers, and improve Medicare benefits.

Democrats could try to raise the Supreme Court itself as an election-year issue, casting the court’s decision as politically motivated.

Top aides at Romney’s Boston headquarters have been working through their potential responses for several weeks. The campaign has sent health care advisers to Capitol Hill in recent days to discuss strategy with Republican lawmakers.

Republicans see political benefits for Romney even if the court sides with the Obama administration.

His repeal refrain could become increasingly attractive to the opponents of the overhaul, and his campaign could make the case that a Romney victory is the next best chance to dismantle a law that’s deeply unpopular with millions of people.

That’s despite Romney’s own support for the “individual mandate” in Massachusetts, where as governor he signed into law a state-based measure that ultimately helped serve as a model for the president’s.

Romney struggled at times during the GOP primary campaign to reconcile his past support for the mandate and current pledge to repeal the federal measure. Obama’s campaign has yet to capitalize with a general election audience on that apparent contradiction.

Romney may face big challenges even if the court strikes down all or part of the law. Attention would turn quickly to what he would do to help 50 million uninsured people get coverage and bring down the nation’s spiraling health care costs.

He has promised to replace the measure with a handful of “common-sense” reforms, but has spent little time so far crafting a comprehensive plan.

“Whether all or part of ‘Obamacare’ will have to be repealed, that’s the first step, and that’s the immediate priority,” said Romney’s domestic policy director, Oren Cass. “His goal will be to repeal anything that’s left of ‘Obamacare’ and actually sit down and identify the challenges we want to address and address them.”

The Obama campaign would seize on Romney’s opposition to the most popular provisions in the law. For example, Romney would not prevent health care companies from denying coverage to new customers with medical conditions. Nor would he force them to cover young adults on their parents’ plans through age 26.

The candidates aren’t the only ones working on their responses ahead of the court’s ruling.

In a memo to fellow Republicans this past week, House Speaker John Boehner warned against “spiking of the ball” if the court strikes down the law.

GOP lawmakers say they quickly would mobilize repeal votes in the House if the court upholds at least some of the law. Similar congressional repeal attempts have been blocked by the Democratic-controlled Senate, and this renewed effort probably would end up the same way.

A congressional Republican aide said GOP lawmakers would try to replace the law, though those efforts would be slower and may not happen this year. The party’s focus would be on controlling costs, not on ensuring coverage for all people, said the aide, who was not authorized to discuss the issue publicly and spoke on condition of anonymity.