ap photo A Frontier Airlines jetliner taxis to a runway to take off from Denver International Airport in 2020. Frontier Airlines’ parent company is buying Spirit Airlines in a $2.9 billion cash-and-stock deal that will allow the combined airline to be more competitive against its larger rivals.

Frontier Airlines and Spirit Airlines are proposing to combine in a $2.9 billion deal that would create a larger discount airline to compete against the nation’s dominant carriers and, they say, promote lower fares.

Both are ultra-low-cost carriers that tempt travelers with rock-bottom prices for no-frills service but often generate more than their share of consumer complaints.

The deal is likely to get a close examination from antitrust regulators in the Biden Administration, which has signaled a tougher line against big corporate mergers. Consumer advocates criticized the Obama administration for allowing a string of major-airline mergers that greatly consolidated power in the industry.

However, the Frontier-Spirit combination would rank only fifth among U.S. airlines in passenger-carrying capacity and seventh in revenue. Frontier and Spirit are pitching their merger as a counterbalance to American, Delta, United and Southwest, which together control about 80 percent of the U.S. air travel market.

“The Biden administration has made it very clear over the last year that they would like to promote competition in the airline space, and this is really an answer to returning balance from a competitive perspective to the big four,” Frontier CEO Barry Biffle said in an interview.

Savanthi Syth, an airline analyst for Raymond James & Associates, said that because of the relatively small size of Frontier and Spirit, she wouldn’t expect antitrust issues “in a normal environment … but given the Biden Administration’s ‘big is bad’ approach we would expect some objection.”

The Biden administration sued last September to block a partnership between American Airlines and JetBlue in the Northeast, saying it would reduce competition and drive up prices. The case is pending.

In antitrust circles, airlines are “certainly an industry where there is a perception that a bunch of mergers got through that maybe should not have,” said Daniel Crane, a University of Michigan law professor and antitrust expert. With the Justice Department already challenging the American-JetBlue deal, “there is a real focus on being more aggressive on mergers,” he said.

Airlines are struggling to recover as the pandemic stretches into a third year. Frontier and Spirit both reported Monday that they suffered fourth-quarter losses — $87.2 million for Spirit, $53 million for Frontier. Both also posted full-year losses for 2021.

The airlines claim that if they are allowed to merge it will create many new routes that aren’t currently served by ultra-low-cost carriers, resulting in $1 billion a year in savings for consumers. They also say the combined company will grow and create 10,000 new jobs by 2026.

Ultra-low cost airlines have shaken the airline industry in recent years, using their lower cost structure — including less-senior workers — to take customers away from entrenched carriers and lure people who balk at paying major-airline fares. Frontier and Spirit say their costs are up to 40 percent lower on a per-mile basis, which will discourage bigger airlines from matching their prices.