WASHINGTON -- 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.”

       Sens. Elizabeth Warren, D-Mass., and Bernie Sanders, D-Vt., along with Rep. Alexandria Ocasio-Cortez, D-N.Y., and five others urged Transportation Secretary Pete Buttigieg and the head of the Justice Department’s antitrust division to review the merger closely for potential violations of antitrust law, and consider acting to stop it.

       The federal agencies declined to comment Friday beyond saying they had received the letter.

       The Biden administration has signaled that it will scrutinize mergers more closely to promote competition. 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 Trump administration had allowed that deal to go through.

       Frontier and Spirit announced last month that they would combine in a $2.9 billion deal to create the nation’s fifth-largest airline by passenger-carrying capacity. Denver-based Frontier would have a controlling share.

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

       The lawmakers, however, said “Spirit and Frontier are already the two least-liked airlines in America,” and a merger could make the customer experience worse because travelers who get mistreated would have no close alternatives among discount carriers.