The bill, the Employee Free Choice Act, would force companies to recognize a bargaining unit when more than half of workers at a site sign cards authorizing one. Under current law, companies can demand a secret-ballot election monitored by the National Labor Relations Board.

Sen. Arlen Specter, R-Pennsylvania, said that he is rejecting the bill because of the so-called card-check provision and another one that would require mandatory arbitration if the employer and union fail to reach agreement on a first contract within 120 days.

Specter’s opposition likely means that all 41 Senate Republicans will stick together in a procedural maneuver that prohibits a final vote. The measure went down in 2007 on such a move, called a filibuster.

At that time, the Senate had 51 Democrats and 49 Republicans. It takes 60 votes to end a filibuster. In 2007, Specter voted against a filibuster.

After last fall’s election, the Senate has 41 Republicans and 58 Democrats, pending the outcome of a disputed race in Minnesota.

The measure was reintroduced on March 10 because it did not become law in the previous Congress. The bill has garnered fewer co-sponsors this time around than it did in 2007, including Democrats who previously supported it.

A fierce lobbying war has erupted over the bill. Organized labor says it is vital for raising workers’ living standards. Business interests say that it robs them of their right to a secret ballot and would raise the cost of doing business.

The top priority of organized labor, the bill likely would sharply increase the number of union members. Currently, 7.6 percent of private-sector workers are organized.

Proponents say that the bill does not eliminate secret-ballot elections. It gives workers the card-check alternative so that they can form unions without being subject to bullying tactics from management.

Opponents of the bill counter that the public signing of cards would make workers vulnerable to strong-arm union tactics. Many union drives are currently decided by card check, when companies acquiesce to the process.

Specter, a moderate Republican who has received labor support throughout his five terms in the Senate, said that there is “widespread intimidation on both sides.”

But he said that the union bill is not the appropriate remedy—nor is it an avenue to ensuring that more workers receive wage and benefit increases through union leverage.

“President Obama has pressed labor’s argument that the middle class needs to be strengthened through more power to unions in their negotiations with business,” Specter said. “The better way to expand labor’s clout in collective bargaining is through amendments to the NLRA [National Labor Relations Act] rather than on eliminating the secret ballot and mandatory arbitration.”

Although they now have a steeper climb to get the bill enacted, supporters are maintaining their resolve.

“We will continue to work with Democrats and Republicans to pass this vital legislation and make the economy work for everyone again,” said Mary Beth Maxwell, executive director of American Rights at Work.

Specter’s decision may force both sides to do something that they say they adamantly oppose—compromise. Recently, three companies—Costco, Whole Foods and Starbucks—formed the Committee for a Level Playing Field and offered an alternative proposal, which was rejected by EFCA supporters.

“We agree with Sen. Specter in his statement today that a third way is needed other than the card-check bill to reform labor laws, one that attempts to level the playing field for both management and labor,” said Lanny J. Davis, an attorney for the committee. “We would be happy to work with him, other congressional members and other parties on this effort in the weeks and months ahead.”

But a business group warned against any proposal that includes the card-check idea.

“We urge all senators to oppose any efforts that would take away the protection of the private ballot, impose binding interest arbitration on America’s small businesses and other employers, or create one-sided remedies,” said U.S. Chamber of Commerce president and CEO Thomas Donohue in a statement.

Specter said that he might give EFCA a second chance down the road.

“If efforts are unsuccessful to give labor sufficient bargaining power through amendments to the NLRA, then I would be willing to reconsider Employee Free Choice legislation when the economy returns to normalcy,” he said.