The rumored merger talks between United and American Airlines could create an unprecedented concentration of power in the commercial aviation industry.

The airline created by merging the No. 1 and No. 2 carriers would control roughly 40% of US capacity when the available seats are adjusted for miles flown. That degree of consolidation would worry rivals, which could be forced to pursue deals of their own.

It could also raise costs for the millions of passengers who book flights on virtually any US airline, according to airline experts.

“The idea that we would have one airline responsible for four out of ten flights every day is beyond horrific,” said William McGee, a senior fellow for aviation and travel at the American Economic Liberties Project, a public interest group concerned with corporate mergers. “It would be harmful to consumers, harmful to labor, harmful to entire cities and regions.”

United Airlines CEO Scott Kirby has approached Trump administration officials about seeking preliminary approval for a merger between his airline and American, according to reports by Bloomberg and Reuters. United declined to comment, and American did not respond to a request for comment. But the reports sent shares of American Airlines climbing.

The US airline industry has been buffeted by mergers ever since it started carrying passengers roughly a century ago.

A slew of mergers in the last 25 years has taken more than a dozen airlines down to only four major carriers – United, American, Delta and Southwest – which together control 80% of the nation’s airline capacity. Two other carriers have capacity just below the big four – Alaska Air and JetBlue.

Those mergers eliminated competition from once-major airlines, including TWA, US Airways, Northwest and Continental, as well as some smaller carriers, such as America West, ATA and Virgin America.

Now, some big US cities have just one airline essentially controlling most of their flights: American has 83% of capacity in and out of Dallas-Fort Worth and 89% in Charlotte, according to a client note from TD Cowen, citing data from aviation analytics firm Cirium. Delta has 77% of capacity in Atlanta and 73% in Detroit. United has 82% of capacity at Washington Dulles airport and 75% at the George Bush Intercontinental Airport in Houston.

That control means they face limited or no competition on many routes in those markets, and thus little pressure to keep prices low.

Because airfare dynamics are complex, it’s difficult to quantify exactly how much consolidation affected ticket prices, said William Kovacic, law professor at George Washington University director of the Competition Law Center there.

“But I think there’s pretty broad agreement that the prices are higher because of the consolidation, and that the consolidation in certain crucial hubs is pretty extreme,” he said.

What a United and American tie-up would mean

Control over America’s travel hubs would grow only more severe should United and American combine, especially in three of the nation’s largest markets: New York, Chicago and Los Angeles. United and American control 46% of capacity in Los Angeles, 45% at the three New York airports, and 70% of the capacity of the two Chicago airports.

That’s why Kovacic doesn’t think a merger between the two would ever be approved, even if the Trump administration were to give it the all-clear.

Even if the airlines convince the Department of Justice, “state governments will still be on the barricades to oppose them, and they have all the means they need to stop the transaction,” said Kovacic. He said foreign governments around the globe would also move to block it using their own antitrust rules.

But McGee is worried that state regulators wouldn’t have the power to block a deal if the federal government signs off. He said federal law gives control over airline regulation to the federal government alone. And foreign regulators could only impact flights to their markets, not domestic US flights.

Even the mere discussion of a United-American deal is likely to bring about negotiations on other deals involving the remaining airlines, said McGee.

That’s especially likely to happen given current economic conditions. Other airlines, especially smaller ones, are looking at deep losses in the months ahead with jet fuel prices roughly double what they were before the war with Iran sent oil prices soaring.

Airlines have often cited financial distress as an argument for consolidation.

Of America’s largest six airlines, only JetBlue hasn’t been involved in a significant merger in the past 20 years, although that’s not for lack of trying. Its deal to buy Spirit Airlines was challenged by the Biden administration and blocked by a federal judge on antitrust grounds in 2024.

The Trump administration seems to be more open to consolidation. Presidents normally don’t take a direct role in approving or rejecting mergers. But when asked earlier this month about whether the government would approve more airline mergers, Transportation Secretary Sean Duffy told CNBC that President Donald Trump would be concerned about reducing competition but “loves to see big deals happen.”

“Who knows who’s going to match up, right?” he added. “But is there room for some mergers in the aviation industry? Yeah, I think there is.”