Twenty-nine years into his U.S. Senate career, Arlen Specter cast what he calls his most difficult vote ever -- a "yes" on the $787 billion economic stimulus bill that made him the only Republican facing re-election in 2010 to support it.

Now, with GOP anger still simmering, Specter is under pressure to buck the party again and support "card check" legislation to make it easier for workers to form unions.

It is only the latest tight spot for the 79-year-old Specter, a political moderate and maverick who is used to being on the political rack, stretched between the wishes of an increasingly conservative party in an increasingly liberal state.

He is in meetings every day about the card check bill, he said, hearing more about it than any other issue and not revealing to anybody which way he is leaning.

"I've been in this line of work long enough that people ... know my arm's not twistable," Specter said in an interview Thursday.

It is that streak of independence that the fifth-term Specter flaunts and Republicans fear.