WASHINGTON — President Obama has shown a willingness to campaign against the Supreme Court if the justices strike down his 2010 health-care law. It’s a strategy that’s as risky as it is rare.

With the court months away from a ruling, Obama ratcheted up the political stakes this week by saying a decision to reject the law and its requirement that Americans get insurance would be “judicial activism” by “an unelected group of people.”

Taking on the court would mean fighting an institution that polls show is historically the most admired branch of government. That’s one reason no major party nominee has made the court a central issue since 1968, when Richard Nixon tapped into voters’ unease about rising crime by attacking the expansion of suspects’ rights under Chief Justice Earl Warren.

“The risk any president faces is that criticism of the Supreme Court can backfire,” said William G. Ross, a constitutional law professor at Samford University in Birmingham, Ala., who has written about the role of judicial issues in presidential campaigns. “People can perceive it as unduly disrespectful of an institution that commands tremendous amounts of public respect.”

Still, the court’s approval ratings have declined in recent years, and there are indications the public sees politics infusing the biggest rulings. In a Bloomberg National Poll conducted March 8-11, 75 percent of respondents said they expect politics will influence the health-care decision, while only 17 percent said they believe the case will be decided solely on its legal merits. Eight percent said they weren’t sure.

Democrats are increasingly questioning the motives of the court and its majority of five Republican appointees. A decision striking down the law would almost certainly be along party-based lines, with the five Republican-appointed justices joining to invalidate the measure and the four Democratic appointees dissenting.

Sen. Patrick Leahy, D-Vt., the Democratic chairman of the Senate Judiciary Committee, said last week that if the court strikes down the law, “it will be a major, major issue in the elections, congressional and presidential, this fall.”

Obama was training his sights on the court even before the health-care case landed there. The president used part of his 2010 State of the Union address to criticize a court decision letting corporations spend unlimited sums on political advertising, saying it would “open the floodgates” for special interests to “spend without limit in our elections.”

This time, he isn’t waiting for the court to rule. Last week’s three-day, 6 1/2-hour argument – the longest in 44 years – suggested the court might turn down at least the core of the health-care law, the insurance mandate. The session at times took on almost a political air as the justices debated whether Congress would be able to re-enact or repeal parts of the law.

This will mark the first time the court has ruled on a president’s signature legislative accomplishment in the middle of his re-election campaign. The decision will probably come in late June, less than five months before the election.

Obama twice this week said he was confident the court will uphold the law, which would expand health insurance to at least 30 million people and reshape an industry that makes up one- sixth of the U.S. economy.

In almost the same breath, he went on the attack, saying a ruling against the law would be the very type of “judicial activism” Republicans have long denounced. The phrase has become a standard Republican line for criticizing Supreme Court decisions backing abortion access, expanding gay rights and limiting the death penalty.

Sen. Mitch McConnell of Kentucky, the top Senate Republican, said in an interview that Obama is “trying to intimidate them into making a decision on Obamacare that he favors.”

In remarks prepared for delivery to the Rotary Club of Lexington, Ky., Thursday, McConnell said Obama “crossed a dangerous line this week” and needs to “back off.”

Republicans have already begun to use the case for campaign purposes, posting an Internet advertisement last week that altered the audio from the argument to attack the law.

Jay Carney, a White House spokesman, Wednesday called the president’s comments “the reverse of intimidation.”

Obama was saying only that he expects the court to uphold the law on the basis of prior cases backing congressional power, Carney told reporters. “He’s simply making an observation about precedent and the fact that he expects the court to adhere to that precedent,” he said, adding that Obama spoke about the case only after being asked a question by a reporter.

Outside observers questioned the wisdom of Obama’s comments at a time when the justices might not have made up their minds. The president risks alienating Chief Justice John Roberts and Justice Anthony Kennedy, the very ones he needs to support the law, said Grier Stephenson, a government professor at Franklin & Marshall College in Lancaster, Pa.

“That he warned the court against doing it would almost be like a challenge to the court, and that might backfire,” said Stephenson, author of “Campaigns and the Court.” “You never know how individual justices might take being warned away from a particular decision.”

Already, at least one Republican-appointed judge is bristling at the president’s comments. A day after Obama spoke, a federal appeals court reviewing a separate part of the health- care measure ordered the Justice Department to submit at least a three-page letter stating whether it believes courts can strike down unconstitutional laws.

Obama’s statement “has troubled a number of people who have read it as somehow a challenge to the federal courts or to their authority,” Judge Jerry Smith told a Justice Department lawyer in court.

More broadly, Obama would run a political risk in attacking an institution that remains popular, though less so than in previous years.

The court’s approval rating stood at 46 percent, with 40 percent disapproving, according to a Gallup poll taken in September. As recently as 2009, the court’s approval rating stood at 61 percent.