Senator Jim DeMint (R-SC) has been a lone voice in blocking the nomination of Errol Southers to head the TSA. His reason for blocking the nomination was that he'd received no assurances that Southers won't implement DHS Secretary Janet Napolitano's plan to unionize the TSA.
This link points out how unionizing the TSA is a security risk.
They should also state the obvious, that unionizing the TSA would make it harder to fire bad employees. That's a security concern.