Technically, Solis' nomination isn't being "held" -- that occurs when a nominee has won committee approval and a senator tries to delay a full vote. Solis has not received a vote yet in the Senate labor committee, chaired by Ted Kennedy (D-MA), but GOPers are slowing down her confirmation over the so-called "card check" bill, a major priority of the labor movement that would allow workers to organize more easily.
Politico had this story first, reporting last week that Solis' hedging response on the card check issue had left GOPers frustrated. And I've got to back them up on this one; the nominee was indeed bizarrely un-forthcoming about her plans on the union organizing proposal.
Instead of defending or criticizing the legislation, she offered a deferral to President Obama and, as Politico notes, told Sen. Lamar Alexander (R-TN) she was "not qualified" to talk about right-to-work laws. There's nothing wrong with taking a politically contentious stand, but Solis didn't take much of a stand at all.