How big is too big for Delta Air Lines?

ATLANTA — As Delta Air Lines settles into its new role as world's largest carrier, it is coming up against a critical question: How big is too big?

In recent months, Delta has sought approval for deals with competitors that would expand its market presence in New York, Australia and Japan — and in some cases run into significant opposition.

Delta became the world's biggest airline through its merger with Northwest Airlines, which closed in October 2008 but is still in progress from an operational standpoint.

Before that, Delta had been the No. 3 U.S. airline, with the Southeast as its traditional stronghold. United and American airlines were bigger.

Delta got Justice Department approval of the Northwest merger under the Bush administration. But observers say the Obama administration is likely to exercise greater scrutiny of cases where Delta is seeking approval for agreements with competitors.

A vocal opponent of Delta's merger with Northwest, House Transportation Committee chairman Jim Oberstar, D-Minn., "is certainly concerned about lack of competition and effects to service as we have more and more consolidation in the airline industry," said spokesman Jim Berard.

"And certainly when the biggest gets bigger, that does raise those concerns."

Delta's proposed partnership with Japan Airlines has drawn questions about whether it would pass antitrust muster and criticism that it could be bad for consumers.

"I don't see (the Department of Transportation) approving antitrust immunity for that at all," said Kevin Mitchell, chairman of the Business Travel Coalition. If it did, Mitchell said he thinks the Justice Department would take action and the U.S. Senate Judiciary Committee would hold hearings on the matter.

Some airline partnerships stop at joint frequent flier benefits or "code-sharing," the practice of selling seats on each others' flights. By gaining antitrust immunity, carriers can coordinate on routes, capacity changes and pricing to control competition.

Industry experts say antitrust immunity granted by the DOT is a key way for airlines to expand globally, amid restrictions on foreign ownership and domestic flying by foreign airlines.

"It gets you to 80 percent of a full-blown merger," Mitchell said.