Now the government’s lawsuit last week to block the merger has put both of those expectations in doubt.

If American cannot grow by merging, it could decide to add flights to better compete with larger rivals. Doing so likely would reduce airfares — and profit margins — across the industry, an outcome that many airline investors fear.

American has faded to a distant third and is particularly weak along the East Coast. US Airways is No. 5 among U.S. carriers, lacks strength in the middle of the country and doesn’t even fly to Asia. But put them together under the American Airlines name, and you’ve got the biggest airline in the world. Right now, both carriers serve Charleston.

Size helps airlines land important corporate-travel accounts. Business passengers will pay higher fares for a carrier that gets them where they want to go. That’s why American and US Airways are vowing to fight the U.S. Justice Department’s lawsuit to block their merger.

“If this merger does not take place, US Air will continue to have the gaps and weaknesses in its network that it has now, and American will continue to have the gaps and weaknesses in its network,” said Joe Sims, an antitrust lawyer hired by American. “Neither individually will be as effective a competitor to United and Delta and Southwest and all the smaller low-cost carriers.”

Airline profits have improved in recent years as mergers eliminated several major airlines and the survivors limited the supply of airplane seats, driving up fares. Many analysts think that if American remains independent, it will pursue a rapid-growth strategy — that was CEO Tom Horton’s plan before the merger. Wall Street didn’t like it.

If American tries to grow to catch up to United and Delta, “the industry’s longer-term earnings prospects are jeopardized,” J.P. Morgan analyst Jamie Baker wrote in a note to clients. Fitch Ratings said the government’s move against the merger could mean the end of airline consolidation, making the lawsuit a negative for the industry’s credit ratings.

American’s parent, AMR Corp., and US Airways Group Inc. announced their merger in February and AMR made the deal part of its reorganization plan that a federal bankruptcy judge was expected to approve on Thursday. The plan would have allowed AMR to end nearly two years in bankruptcy.

But on Tuesday, the Justice Department and six states sued to block the deal, saying it would hurt competition and drive up prices for consumers. On a conference call with reporters Wednesday, lawyers for the airlines said they were confident of winning when the case goes to trial. They said that the government has stopped only one merger in the past eight years.

“We were surprised that the (lawsuit) wasn’t stronger,” said one of the lawyers, Paul Denis. “We usually expect the government to put on a better case.”

Justice Department spokeswoman Gina Talamona declined to address the lawyers’ specific comments but defended the lawsuit. “We allege that the deal will lessen competition, and passengers will pay higher fares and receive less service,” she said.

Executives for American and US Airways have declined to say yet what they would do if the merger is blocked.