In January, airline passengers who use wheelchairs saw brighter horizons ahead for their often-difficult travels. A new rule adopted by the federal government meant that airlines would expand support for disabled passengers throughout their trips and enhance training for employees who assist them, and carriers would be compelled to replace wheelchairs that were lost or damaged and offer loaners promptly, among other changes.

But shortly after President Trump was inaugurated, the Transportation Department moved to delay enforcement of the rule — initially until March, then until August — and now, airlines are challenging one of its provisions in court.

When the government issued the rule in December, supporters heralded it as a win after decades of advocacy, saying it struck a blow for the rights of disabled travelers, for whom inconsistent help, physical discomfort and damaged wheelchairs were unfortunate norms of flying.

“Those rules were a huge win for wheelchair users,” said Seth McBride, a wheelchair rugby athlete who lives in Washington State. “It felt like the D.O.T. was starting to listen to the disability community,” he said, adding that he felt that airlines were being forced to take the needs of wheelchair users seriously and provide the same level of service they provide to everybody else.

Now the rule is on notably shakier ground. Airlines for America, a trade association that represents the country’s biggest airlines, joined by member carriers American Airlines, JetBlue Airways, Southwest Airlines, United Airlines and Delta Air Lines, has filed a lawsuit in federal court arguing that the Transportation Department overstepped its statutory authority in adopting the measure.

The petitioners argued in an opening brief filed last week that the rule wrongly makes airlines liable for damaged wheelchairs even when they have not discriminated against disabled travelers, such as if mishandling was an “act of God” or beyond the airline’s control. One example they cited was if the wheelchair was in poor condition before the start of the trip. In those cases, they should not be held liable, they said, allowing that the rest of the rule’s provisions could be left in place. The lawsuit “simply seeks to ensure that D.O.T. acts within the scope of its authority to regulate only acts within airlines’ control and responsibility,” the brief reads.