"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.

American Airlines, which Delta is trying to unseat as JAL's U.S. ally, has argued that it is the better partner because it is more likely to get antitrust immunity and its deal is pro-competitive.

Delta Chief Executive Richard Anderson has said American is focusing on regulatory issues rather than on which deal is better in the long-term for business.

Antitrust immunity between Delta and Japan Airlines could lead to "much, much higher prices," Mitchell said.