WASHINGTON  Memo to the third floor at Justice Department headquarters: Ian Gershengorn will soon be pacing the corridors again.

When preparing for oral arguments in big cases, Mr. Gershengorn makes a habit of walking long laps around the building’s rectangular core, first past the civil and antitrust divisions, then the Office of Professional Responsibility and the appellate branches, mumbling to himself while hunched over note cards. People try not to stare.

There has been a lot of pacing lately. Since March, it has fallen to Mr. Gershengorn, 43, a deputy assistant attorney general, to defend the Obama administration against nearly two dozen legal challenges to the president’s health care overhaul. For the moment, the burden of defending the constitutionality of President Obama’s signature legislative achievement falls squarely on Mr. Gershengorn and his team of more than a dozen litigators.

The health care cases have turned Mr. Gershengorn into something of a courthouse circuit-rider, traveling week after week to Virginia and then Florida and on to Michigan to repeat the same arguments.