“It’s impossible to underestimate just how important the effect of low-cost carriers are on a given route,” said William McGee, the aviation adviser for Consumers Union.

Carriers like United and American do not compete with carriers like Frontier and Spirit on every type of passenger. Lucrative corporate accounts are owned by the big carriers, and business travelers avoid the cheaper airlines, often choosing to pay premium prices at the last moment to get seats on the flights that best fit their schedules.

But the low-cost carriers nonetheless force the big airlines to figure out a way to draw the most price-sensitive fliers in any given market — those who scour the internet for the cheapest tickets possible. Those customers make up a significant portion of travelers, meaning the major carrier cannot just ignore them.

“Those passengers certainly are important,” said David Weingart, an economist at the aviation consultant GRA. “The larger airlines have proven that in how they’ve reacted, in how they’ve tried to capture or recapture those passengers.”

Delta, American and United Airlines have all rolled out “basic economy” fares. Such tickets are priced competitively against Spirit and Frontier, but do not offer the amenities that most consumers have come to expect on a flight, like receiving a seat assignment ahead of a flight or obtaining a refund for a ticket.

Of all the major carriers, United is fighting on price the most aggressively.

Scott Kirby, who was appointed as United’s president a year ago, has shifted the carrier’s strategy toward the low-cost airlines, mirroring one he helped to drive when he served as a top executive at American.

Pushing back against Wall Street’s wishes to limit capacity growth, United is adding seats in a number of its major markets across the country. It has, for example, swapped out smaller jets for larger planes to increase the number of seats it has available to sell, and matched fares offered by low-cost carriers.