Sixteen Senate Republicans on Wednesday called on congressional leaders to extend a program that provides money to support airlines’ payrolls in exchange for prohibiting them from laying off employees during the coronavirus pandemic.

The $25 billion payroll support program was passed as part of March’s massive economic relief package known as the Cares Act and is set to expire on Oct. 1. Airlines have warned that they might have to lay off more than 74,000 employees after that date as they continue to experience deeply depressed demand for flights due to the pandemic.

In a letter to colleagues, the senators wrote that they “support a clean extension of payroll support for passenger air carrier employees included in the Cares Act to avoid furloughs and further support those workers.”

The letter’s lead signer was Sen. Cory Gardner (R-Colo.), and it was also signed by Sen. Roger Wicker (R-Miss.), who chairs the committee that has responsibility for aviation.

The senators join more than 223 members of the House, mostly Democrats, but also a few Republicans, in signaling their support for the program, boosting its chances of being included in a final deal on a second round of coronavirus relief.

Securing the program has been a priority for labor unions representing workers in the aviation industry. Airline leaders have said they support the idea, while emphasizing that the effort is being led by the unions.

Sara Nelson, international president of the Association of Flight Attendants-CWA, which represents 50,000 flight attendants at 19 airlines, said her members had “burned up congressional phone lines over the past week” to obtain support for extending the program.

“This provision is the most successful jobs program of covid relief and maintains service to all of our communities,” Nelson said in a statement.

The Cares Act was passed before the country went into its period of tightest lockdown to slow the spread of the novel coronavirus, and air travel all but ceased. But neither Democrats in the House nor Republicans in the Senate included an extension of the airline payroll program in their opening proposals for the next round of relief.

Democrats passed their version in May, when it looked as though demand for air travel might recover well before October. But as the virus has persisted in the United States and some governors have required visitors from hot-spot states to quarantine, the growth in passenger travel has stalled at about 30 percent of last year’s levels. On Tuesday, the Transportation Security Administration screened 543,000 people, compared with almost 2.4 million on the same day last year.

Airline executives say they expect a long, uneven recovery and that the industry will be smaller for years to come. Jobs are not under threat across the industry: Southwest Airlines has said it doesn’t anticipate layoffs. But United Airlines has warned of the largest layoffs, saying some 36,000 employees could be let go. American Airlines has warned of about 25,000 layoffs.

Joe DePete, president of the Air Line Pilots Association, International, said Congress must act to save those jobs.

“Unless we all act now, this aid, and the strong labor protections attached to it, will expire Oct. 1 — even though the virus is not under control and the travel industry remains devastated,” he said in a statement.

Lori Aratani contributed to this story.