Frontier Group Holdings Inc.’s proposed acquisition of Spirit Airlines Inc. may increase air fares at the time when inflation in the U.S. is already at record high, Senate Democrats warned, urging a review of the deal.

“There are serious risks that a Spirit-Frontier combination would further concentrate and even monopolize the ultra-low-cost carrier segment,” Senators including Elizabeth Warren said in a letter to U.S. Assistant Attorney General Jonathan Kanter and U.S. Transportation Secretary Pete Buttigieg.

Demands from Democrats put to the test the White House antitrust agenda. President Joe Biden sees consolidation and lack of competition as one of the main reasons for higher prices and has urged higher oversight of the airline industry. Still, the deal might get clearance, because the networks of the two airlines are mostly complimentary, according to Bloomberg Intelligence analysts.