WASHINGTON — Giving a surprise boost to the new health care law, one of the nation’s most closely watched federal courts ruled Tuesday that the law’s requirement that most Americans get health insurance is constitutional.

The split opinion by the conservative-leaning U.S. Court of Appeals for the District of Columbia marks the second time this year that a federal appellate court controlled by Republican appointees has backed the law and its insurance mandate.

And though the ruling has little practical impact, it comes just as the U.S. Supreme Court begins considering the constitutionality of the landmark legislation that President Barack Obama signed last year.

With the opinion, three federal appellate courts — in Washington, in Richmond, Va., and in Cincinnati, have now rejected substantive challenges to the health care law. Only the 11th U.S. Circuit Court of Appeals in Atlanta backed such a challenge.

The case decided Tuesday was a challenge to the law’s insurance requirement by the American Center for Law and Justice, a conservative legal group founded by evangelist Pat Robertson.

The suit was dismissed by a federal judge earlier this year. But many believed the three-judge panel of the D.C. circuit, which included two Republican appointees, would reverse the lower court and uphold the challenge to the law.

One of the GOP appointees — Judge Laurence H. Silberman — was appointed to the bench by President Ronald Reagan and is considered a conservative intellectual leader on the court. He won plaudits from gun rights groups recently for writing an opinion that the District of Columbia’s handgun ban was unconstitutional.

But in a concise majority opinion in the health care case, Silberman categorically rejected the central Republican attack on the health care law’s expansion of federal regulation of health care.

“The right to be free from federal regulation is not absolute and yields to the imperative that Congress be free to forge national solutions to national problems,” Silberman wrote. He was joined in the opinion by Judge Harry Thomas Edwards, an appointee of President Jimmy Carter.

Silberman and Edwards acknowledged that the insurance mandate, which will require most Americans to get insurance starting in 2014, is unprecedented.

But they broadly accepted the Obama administration’s argument that health care presents issues for a society unlike any other.

“The health insurance market is rather a unique one, both because virtually everyone will enter or affect it, and because the uninsured inflict a disproportionate harm on the rest of the market as a result of their later consumption of health care services,” they wrote.

In a dissenting opinion, Judge Brett Kavanaugh, an appointee of President George W. Bush, said the court can’t review the health care mandate until it takes effect in 2014, a position also taken by the federal appellate court in Richmond.