An effort to privatize air traffic control backed by the White House and Republican leadership has hit a patch of turbulence but still appears on schedule for a congressional vote this year.

The powerful appropriations committees have both rejected bids to attach the proposal to spending bills, but that hasn't undermined an effort to pass separate legislation, say both supporters and critics of the effort.

House Transportation and Infrastructure Committee Chairman Bill Shuster, R-Pa., is leading the effort, which would take air traffic control out of the Federal Aviation Administration and instead create an independent nonprofit to run it. It would be governed by a board including the transportation secretary, people nominated by the airline companies and representatives of the air traffic controllers' and pilots' unions. The board would be funded through user fees. Shuster and other fans argue a non-government entity would be better suited to keep the technology up to date.

Support for and opposition to the effort cut across partisan lines. President Trump has backed it, as has House Speaker Paul Ryan, R-Wis. The main aviation industry lobby, Airlines for America, is pushing for it, as is the National Air Traffic Controllers Association, the labor group that represents the workers. However, liberal and conservative lawmakers have objected, fearing the legislation would give the industry too much power.

Last month, the respective appropriations committees stripped the privatization language from the bill to fund the Transportation Department, stopping one of the proposal's best chances of getting passed. However, the same proposal is expected to be included in separate legislation to reauthorize the FAA after Congress returns from its August break. Transportation Committee spokesman Jeff Urbanchuk told the Washington Examiner, "It's the usual process of legislating. We remain confident in our bill."

Defeat in the appropriations committees wasn't surprising, given that controversial provisions are usually shed to secure passage of spending bills, a congressional source who requested anonymity noted. Heavy industry lobbying and the leadership's support — earlier this year Ryan said "we have to overhaul the entire air traffic control system" — still weigh heavily in its favor.

Airlines for America concedes the effort is facing facing serious resistance: "It would be unfortunate for political turf wars in Washington to stand between American families and businesses and the modern, 21st -century [air traffic control] system they are paying for but haven't yet received," said spokesman Vaughn Jennings, adding that, "We remain confident that Congress will seize this opportunity."

Critics are hoping that momentum for it is slowing. "Delaying the FAA reauthorization vote until after the recess shows that many members are studying this issue and coming to the same conclusion that I have: ATC privatization is a threat to our national security. I'm glad we're having this debate because we certainly must modernize our ATC system, but we can do that without turning over our nation's air space to a private corporation," said Rep. Ralph Abraham, R-La., one of the proposal's leading critics.

The FAA has been attempting modernize and replace its aging radar-based air traffic control system for years with a GPS-based system dubbed NextGen. Shuster and others argue that moving the the system out of government control would speed up that process. Critics argue the FAA is making strides in updating the system.