WASHINGTON — President Trump proposed Thursday to move air-traffic controllers out of the Federal Aviation Administration and to a private corporation, a top priority for most airlines while still contentious in Congress.

The main reason airlines, the controllers’ union and congressional advocates want the change is to avoid annual spending disputes and worker furloughs in recent years.

More stable funding is needed, according to the advocates, to spur the FAA’s multi-year modernization program called NextGen, which is upgrading ground-based radar to satellite-based GPS to track and guide planes.

Greater precision is expected to save fuel, make arrivals and departures more reliable and allow more planes to fly closer together as the number of travelers grows.

But skeptics of privatization contend that the FAA is making progress on NextGen, and that separating air-traffic control could complicate a system that is the safest in the world.

The proposal is part of Trump’s $1.2 trillion discretionary budget blueprint for the year starting Oct. 1. The Transportation Department proposes to start moving controllers “to an independent, non-governmental organization, making the system more efficient and innovative while maintaining safety.”

“This would benefit the flying public and taxpayers overall,” the budget states.

But the impact on government spending isn’t clear because the private corporation is expected to be funded by the user fees corresponding to the federal taxes that airlines already pay the government.

Airlines have campaigned for the change for years to modernize the system faster. The National Air Traffic Controllers Association, the union representing controllers, also supported the move as a way to stabilize funding.

“This is a bold step that will lead to the governance and funding reforms needed to move our air traffic control infrastructure into the 21st century,” said Nicholas Calio, CEO of Airlines for America, a trade group representing most of the large carriers. “Our system is safe, but it is outdated and not as efficient as it should – or could – be. We need to stop accepting pockets of progress and put in place a modernized system that better serves the traveling and shipping public.”

Doug Lavin, vice president for North America of the International Air Transport Association, which represents 265 airlines worldwide, said modernizing the system is crucial as the number of passengers is projected to increase by 500 million per year by 2035.

“Now is the time to move forward with transformation in the US through the creation of a separate, corporatized non-profit entity to manage U.S. skies.”

House Transportation Committee Chairman Bill Shuster, R-Pa., sponsored legislation last year to move air-traffic control to a corporation governed by industry stakeholders, modeled on Canada’s system.

“By removing the (air-traffic control) function from the FAA, Americans will see a more efficient system, flight times decrease, on-time departures increase, emissions reduced, and 21st century technology deployed to guide our planes from gate to gate," Shuster said. "For too long, the federal government has been the impediment in updating our ATC operation to a world-class, state of the art system."

Shuster anticipated opposition, which prevented votes on his proposal last year in the full House or Senate. But Trump's proposal rejects the status quo and will disrupt the old way of thinking, Shuster said.

“Like any transformative change in Washington, entrenched interest groups will do and say anything to protect their parochial interests," Shuster said. "But the facts are not on their side."

Critics such as the top Democrat on the House panel, Rep. Peter DeFazio of Oregon, worry that the shift would make things more complicated because the corporation would still have to ask the Transportation secretary to increase fees, change flight paths or close towers.

“If they disagree, they go to court," DeFazio said. "That’s a disaster in the making as a way to run a vital public service subject to frequent litigation because one airline says your favoring another airline."

Another concern is how the new organization would generate about $11 billion a year that FAA has for air-traffic control, which is now raised mostly through airline ticket taxes, DeFazio said. The private organization is expected to create user fees that replace the taxes, although who would pay and how much must still be determined.

“This whole thing is fraught with uncertainty and problems," DeFazio said.

The top Democrat on the Senate committee that oversees the FAA, Sen. Bill Nelson of Florida, has said privatization was opposed by the Defense Department and would be costly and disruptive.

“This scheme would upset the partnership between the FAA and a lot of the agencies of government,” Nelson said at Transportation Secretary Elaine Chao’s confirmation hearing Jan. 11.

Delta Air Lines broke with most other carriers to oppose privatization at a time when the FAA is making improvements.

FAA Administrator Michael Huerta told the aviation summit March 2 that NextGen has already delivered $2.7 billion in benefits and is projected to provide $160 billion in benefits by 2030.

The leaders of the Senate Appropriations Committee, which decides with House counterparts how much to spend on agencies such as FAA each year, raised concerns about privatization.

“It does not appear to make sense to break apart the FAA, an essential part of our success in aviation,” Sens. Thad Cochran, R-Miss., and Patrick Leahy, D-Vt., said in a letter Feb. 28. “A privatized system would provide consumers with no recourse for complaints or mistreatment, as it current does through the U.S. Department of Transportation or their members of Congress.”