There is also overlap on a few dozen routes in the West, where the carriers operate a collection of hubs and focus cities.
Labor issues are daunting, as well. Integrating Delta's largely nonunion workforce with Northwest's heavily unionized one might appear to be a reason for concern, but it pales in comparison with integrating unionized workers at United and US Airways.
In particular, US Airways pilots are bitterly divided following the 2005 merger with America West, which led to a controversial seniority ruling that placed hundreds of America West pilots with a few years at the company ahead of pilots from the former US Airways with 15 years or more.
The dissatisfaction led to replacement of the Air Line Pilots Association, which embraced the ruling when it might have backed away, with a new union, the U.S. Pilots Association. While backed by most of the 3,500 former US Airways pilots, USAPA has virtually no support from the 1,800 America West pilots.
Understandably, United's 7,000 pilots are put off. "US Airways' pilot integration problems have created a toxic stew," said Steve Wallach, chairman of the United ALPA chapter, in a prepared statement.
A merger would likely bring another election. "ALPA would get a second shot," said aviation consultant Daryl Jenkins. "You don't get too many second chances in this world, but this might be one."
USAPA spokesman Scott Theuer says the potential merger is an unwarranted distraction, because a new federal law requires seniority issues between two companies with different unions to be resolved before a new representational election.