“The complaint went on at length that US Air was a potent competitor because of one stops and Advantage Fares,” Professor Hemphill said. “This made US Air an unusual and potentially disruptive carrier. If that’s true, as the complaint alleged, you can’t divest your way out of that problem. The way the complaint was structured suggested the government would see this to the end and block the merger.”

Professor Hemphill noted that the complaint read as though the Justice Department had come to believe the earlier mergers were a mistake. As the complaint put it: “Increasing consolidation among large airlines has hurt passengers. The major airlines have copied each other in raising fares, imposing new fees on travelers, reducing or eliminating service on a number of city pairs, and downgrading amenities.”

Enforcement of antitrust policy is a law enforcement function, and like other enforcement activities of the Justice Department, it is supposed to be strictly insulated from political pressure. The Tunney Act, which requires judicial review of any antitrust settlement, was passed in 1974 as a result of a scandal in which President Nixon ordered the Justice Department to settle an antitrust case against International Telephone and Telegraph after the conglomerate made secret contributions to the Nixon campaign.

The president was captured on tape calling for the head of the antitrust division, Richard McLaren, to be fired if necessary.

“The I.T.T. thing — stay the hell out of it. Is that clear? That’s an order.” Nixon went on, “I do not want McLaren to run around prosecuting people, raising hell about conglomerates, stirring things up.”

The law didn’t stop American and US Airways from mounting a huge lobbying and public relations campaign. Last month, Rahm Emanuel, now Chicago’s mayor and formerly President Obama’s chief of staff, along with the mayors of Philadelphia, Phoenix, Dallas, Fort Worth, Miami and Charlotte, N.C., signed a letter calling on Mr. Holder to drop his opposition to the merger. Sixty-eight Democratic members of Congress wrote a similar letter to President Obama.