WASHINGTON -- In a setback for organized labor, Pennsylvania Sen. Arlen Specter said Tuesday he will oppose a bill that makes it easier for workers to form unions. 

Specter was the only Republican to support the Employee Free Choice Act two years ago and unions were hoping he might be the crucial 60th vote needed to overcome an expected GOP filibuster of the measure when it's taken up this summer. 

In a floor speech, Specter called it a "very emotional issue with labor looking to this legislation to reverse the steep decline in union membership and business expressing great concern about added costs, which would drive many companies out of business or overseas." 

Specter said his vote to end debate on the bill two years ago was not support for the merits, but instead for Congress to take up the issue of labor law reform. 

The bill would allow a majority of employees at a company to organize by signing cards, a change from current practice that allows employers to mandate secret ballot elections. It also would boost penalties for retaliation against workers seeking to organize and calls for arbitration if management and the union cannot agree on a first contract. 

Specter called the secret ballot "the cornerstone of how contests are decided in a Democratic society." And he said the requirement for mandatory arbitration may subject employers to a deal they cannot live with. 

His decision will make it difficult for Democratic leaders to move forward with the bill, which unions consider their No. 1 priority, but some business groups have labeled "Armageddon." 

Specter has faced unusually heavy pressure from both groups as he faces re-election next year. 

"It is very hard to disappoint many friends who have supported me many years on either side who are urging me to vote their way," Specter said. 

He also said his announcement "should end the rumor mill that I have made some deal for political advantage." 

National Association of Manufacturers president John Engler praised Specter's decision. 

"We are commending Sen. Specter for putting American jobs first and opposing card check legislation," Engler said.