Partisan disputes over airline union provisions, along with the shortened holiday congressional calendar and distractions related to the high profile payroll-tax break debate, have dramatically reduced the likelihood that a multi-year FAA reauthorization will pass by next month’s deadline, once again raising the specter of another FAA shutdown.

The largest remaining obstacle stems from the 2010 rule change by the National Mediation Board that would make it easier for airline workers to unionize. The House version of the FAA reauthorization seeks to reverse the rule, while the Senate version would allow it to remain in place. The disagreement has been so polarizing that the issue is the subject of direct negotiations by speaker of the House, Rep. John Boehner (R-Ohio) and Senate majority leader, Sen. Harry Reid (D-Nev.).

According to the Wall Street Journal, in a potential compromise, elements of which have been circulating on Capitol Hill for months, the House would agree to remove language related to union organizing and deal with the issue in a separate piece of legislation. Many congressional staffers have expressed optimism about the momentum of these high-level negotiations, but stress that the situation remains fluid and no final agreements are in place.

Additionally, issues with other parts of the legislation, such as Essential Air Service (EAS) and shipping requirements for lithium batteries could sink the deal.

The current authorization bill, the 22nd extension since the expiration of the last long-term authorization, is set to expire on Jan. 31.