Specter was a potential swing vote for the measure, so his opposition is a blow to organized labor which has spent millions on garnering support in Congress for the measure which would make it easier for employees to form a union.

It is a win for the U.S. Chamber of Commerce which has also spent millions to oppose the bill.

"The U.S. Chamber of Commerce will continue to use every tool at its disposal to fight this legislation until it is taken off the table altogether," said Thomas Donohue, president and CEO of the chamber.