Congressional leaders reached a breakthrough compromise on Friday on the most difficult issue holding up action on long-stalled U.S. aviation legislation that would accelerate modernization of the aging air traffic system.

Leaders in the Senate and House of Representatives agreed to modify politically charged language on how airline unions conduct their elections, partly meeting Republican demands to reform U.S. oversight of representation balloting, congressional aides said.

The surprise deal brokered by Senate Majority leader Harry Reid and House Speaker John Boehner set the stage for a temporary funding extension of Federal Aviation Administration (FAA) programs that must be in place before any larger bill can be finalized.

The current temporary measure fully funding FAA programs expires January 31.

Agreement on the labor issue was cited by key lawmakers and aides as proof both parties could overcome partisan differences to get work done in a sharply divided Congress.

"Every issue does not have to be a fight. This is a good example of the common sense results that Democrats and Republicans can produce when they work together, and put the interests of the American people ahead of scoring political points," Reid said in a statement.

The airline industry is heavily unionized, and labor groups are pressing to boost their presence at Delta Air Lines, JetBlue Airways and a few small carriers.

The compromise would also tighten U.S. oversight of representation balloting, another Republican condition.

Democrats said they had prevented a broader roll-back of labor law pushed for months by airline allies in Congress.

The deal was pressaged by a recent federal court ruling that effectively rejected airline industry claims on the labor issue.

Other issues remain on the table in aviation bill negotiations but sources with knowledge of the talks have said they could be resolved once the labor provision was settled or set aside.

Inaction since 2007 on the legislation laying out FAA policies and spending has required that Congress approve a string of temporary measures to keep the agency overseeing air traffic services fully funded.

Reid said the deal set the stage for a long-tern measure that would give lawmakers more time to reach terms on the four-year, multi-billion dollar FAA bill.

Sharp political wrangling over the labor issue and another involving subsides for rural airline service triggered a two-week shutdown of airport construction programs last summer.

The subsidy issue is still outstanding, as is another involving airline access rights to Washington's Reagan National airport.

The central provision of FAA legislation would take the next steps in overhauling the nation's air traffic networks from an aging radar-based system to one relying on satellites.