GOP senators say they have more than enough votes to block the measure. Senate Democrats would need 60 senators to avoid a filibuster of the bill. The original bill that created TSA did not allow workers to unionize, reasoning that the administrator needed flexibility to respond to emergencies like the failed bombing plot in London last August, and a union would take away that flexibility.
I happen to think that labor unions are not necessarily a bad thing, as long as they provide a measured counterbalance to what could become unfair management practices. I am less enthusiastic when unions serve to protect incompetent workers or stifle sound and reasonable work practices. S 4 has many important provisions that need to become law, including funding for Emergency Communications Operability and Interoperable Communications efforts and changes in the visa waiver program.