Republican Sen. Arlen Specter dealt a blow to organized labor's top legislative priority by announcing that he wouldn't support a bill to make it easier to unionize workplaces.

The AFL-CIO and the bill's Democratic sponsors had been counting on Sen. Specter's vote to reach the 60 needed in the Senate to avoid a Republican-led filibuster. In 2007, Sen. Specter voted for cloture on the bill, or to cut off debate, but he said he could not do so now, especially given the recession and the weak economy.

"The...