It's a setback for organized labor, and a risky position to take before a re-election bid.
Pennsylvania Sen. Arlen Specter said he will oppose a bill that makes it easier for workers to form unions.
Specter, who faces re-election next year, was the only Republican behind the bill two years ago.
Union organizers had hoped he would be the crucial vote needed to overcome a GOP filibuster when the matter is taken up again this summer.
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