Indeed, concern about concentration was a factor in the Justice Department's review of the failed UAL/US Airways deal. In a 2005 speech outlining how Justice vets airline mergers, J. Bruce McDonald, then deputy assistant attorney general in the antitrust division, said Justice opposed the deal in part because it "increased concentration in large business centers along the east coast" and "would have lessened competition in several trans-Atlantic markets."

One factor favoring consolidation is that the market has changed significantly since 2001. Low-cost carriers such as Southwest Airlines, JetBlue, and AirTran Airways account for a much larger percentage of the domestic market than they did in 2001. At the same time, an agreement between the United States and European Union that takes effect this year promises to open airports and lower airfares for travel between U.S. and European cities that were historically restricted by pacts with a single country. Historically, for instance, only PanAm and TWA could fly into London's Heathrow airport, the rights purchased by American Airlines and United.

The agreement, hailed as "historic" by the Department of Transportation, means lower barriers to entry that will give the international dimension of these deals a better chance of clearing a review by the Justice Department.

"All of the important issues are international; none of the important issues is domestic," says Hubert Horan, an airline consultant who worked on the international alliances, but has authored a paper opposing the current wave of airline mergers. "The whole objective of this is to reduce the U.S.-to-continental-Europe market to a two airline oligopoly."

Kevin P. Mitchell, chairman of the Business Travel Coalition, who spent 14 months convincing regulators to oppose the UAL/US Air deal, would not comment on the deals until they are formally announced. But he said, "I will weigh in strongly against industry consolidation. If the Delta/Northwest deal happens, the others are going to find their own dance partners and we are quickly going to go down from six major carriers to three or four."

The airlines are trying to seize the moment with the Bush administration's antitrust division, according to conventional wisdom. But antitrust lawyers say there might not be enough of a window of opportunity. "Time is running out on these things. The review of an airline deal is not a short, quick thing to do," says one lawyer in private practice.

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