Spirit brings greater international visibility, with nearly three times more overseas flights than Frontier, according to Cirium. The airlines said that together they would be able to serve destinations that one or both had dropped, including Jackson, Miss; Birmingham, Alabama; and Dulles International Airport near Washington. They said the merger could allow the new airline to launch flights to smaller cities as well, including Eugene, Oregon; Ithaca, NY; and Worcester, Mass.

The airlines have argued the deal will benefit consumers, with flights to and from 145 destinations in 19 countries. In November, the average price for a domestic ticket sold by Spirit was $109, before taxes and fees, compared to $73 for Frontier, according to Cirium. By joining forces, the airlines say they will be able to offer more flights on existing routes, giving customers more choice and allowing the new company to better respond to disruption.

“I think it’s a slam dunk, not a reduction in competition,” said industry analyst and consultant Robert Mann. “It basically reinforces the price discipline that the DOJ relies on when allowing other things that are arguably not so good.”

The tie-up would consolidate the airlines’ hold on some airports, which could put pressure on other carriers, such as JetBlue, Alaska Airlines, Hawaiian Airlines and Allegiant Airlines, to join forces through partnerships or of mergers. Combined, Spirit and Frontier would have a 26% market share in Orlando, Florida, more than any other airline, according to Cirium data for 2021. In Las Vegas, the combined carrier would have a 24% share, second only to South West Airlines.

Still, competition in these cities is fierce and nowhere near as limited as in some of the airport hubs serviced by larger carriers, Spirit chief executive Ted Christie said in an interview.

“These are both large leisure destination markets and very competitive,” he said.

American Airlines, based in Fort Worth, has more than 80% of the Dallas-Fort Worth International Airport market, according to data from Cirium. At Hartsfield-Jackson Atlanta International Airport, where Delta Air Lines is based, this airline has 78% of the market. United and Southwest also own similar shares in some of their hubs.

In addition to regulatory approval, Spirit and Frontier will have to renegotiate contracts with their unions, which were notified of the agreement on Monday. Pilots of both airlines are represented by the Air Line Pilots Association, while flight attendants of both are represented by the Association of Flight Attendants.