Spirit brings more international exposure, with nearly three times as many flights abroad as Frontier, according to Cirium. The airlines said that together they would be able to serve destinations that one or both had abandoned, including Jackson, Miss.; Birmingham, Ala.; and Dulles International Airport near Washington. They said the merger could enable the new airline to start flights to small cities, too, including Eugene, Ore.; Ithaca, N.Y.; and Worcester, Mass.

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

“I think it’s a slam dunk, not a reduction of competition,” said Robert Mann, an industry analyst and consultant. “It essentially reinforces the price discipline that D.O.J. relies on when they allow other things which arguably aren’t so good.”

The combination would consolidate the airlines’ hold over 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 mergers. Together, Spirit and Frontier would hold a 26 percent share of the market in Orlando, Fla., more than any other airline, according to Cirium data for 2021. In Las Vegas, the combined carrier would have a 24 percent share, second only to Southwest Airlines.

Still, competition in those cities is fierce and not nearly as limited as in some of the airport hubs maintained by the largest carriers, Ted Christie, Spirit’s chief executive, said in an interview.

“Those are both big leisure destination marketplaces and very competitive as it is,” he said.

American Airlines, which is based in Fort Worth, holds a more than 80 percent share of the market at Dallas-Fort Worth International Airport, according to Cirium data. At Hartsfield-Jackson Atlanta International Airport, where Delta Air Lines is based, that airline holds a 78 percent share of the market. United and Southwest also command similar shares at 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 deal on Monday. Pilots at both airlines are represented by the Air Line Pilots Association, while the flight attendants for both are represented by the Association of Flight Attendants.