WASHINGTON (AP) — At the heart of the bitter dispute over funding the Federal Aviation Administration is an ongoing brawl between Republicans and Democrats over the rights of labor unions, one of several that have flared up during the Obama administration.

Congress struck a deal that gave the FAA temporary operating authority last week just as lawmakers began their August vacation. For nearly two weeks prior, the FAA was mostly shut down, with 4,000 FAA workers furloughed and tens of thousands of construction-related jobs idled. The unsettled labor dispute is waiting for Congress when it returns to consider a long-term FAA spending bill and could lead to another standoff.

House Republicans are intent on overturning a new rule passed by a little-known agency last year that made it easier for airline and railroad workers to unionize. Senate Democrats, meanwhile, have made clear they want to keep the new rule, one of the few big victories for organized labor under the Obama administration.