Spirit and Frontier say their merger would help consumers by creating a more powerful low-cost competitor to American, Delta, United and Southwest.

The merger of Frontier and Spirit would create a low-cost competitor to American, Delta, United and Southwest. The photo is from Spirit Airlines.

Several congressional Democrats are warning that the proposed combination of Frontier Airlines and Spirit Airlines would reduce competition among low-cost carriers and drive up fares during a time of inflation.

The lawmakers said previous mergers have hurt consumers and workers, and if Frontier and Spirit ever drop their budget-carrier business model, the industry would lose an important check on prices.

The heads of the Justice Department's antitrust and transportation departments were urged by a group of people.

The agencies said they had received the letter.

The Biden administration will scrutinize mergers more closely. In September, the Justice Department sued to block a partnership in which American Airlines and JetBlue Airways agreed to work together in the Northeast. The deal was approved by the Trump administration.

Frontier and Spirit announced last month that they would combine, creating the nation's fifth-largest airline by passenger-carrying capacity. Denver-based Frontier would have a controlling share.

The deal would help consumers by creating a more powerful low-cost competitor to American, Delta, United and Southwest, which together control 80% of the U.S. air travel market.

The lawmakers said that a merger would make the customer experience worse because travelers who get mistreated would have no close alternatives.

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Democrats are against theSpirit-Frontier-merger.