In 2007, the House passed a similar bill, but it failed in the Senate on a procedural vote.

Republican leaders and business lobbyists say the Democrats do not have the 60 votes to overcome a filibuster. But union leaders voice optimism, noting that Mr. Obama has endorsed the bill and that Democrats have close to 60 seats in the Senate, though two remain in dispute. Arlen Specter, a Pennsylvania Republican who once was a co-sponsor of the bill, has not decided whether he would support it this time, an aide said.

Whether it is Wal-Mart or the National Restaurant Association, many companies and corporate groups financing the opposition fear that their companies and industries will be among labor’s earliest organizing targets should the bill become law.

Labor leaders say they are setting their sights on several industries, like banks and big-box retailers like Wal-Mart or Target, where unions have had virtually no success.

Image Workers at the Smithfield plant in Tar Heel, N.C., in 2006. They voted to unionize last month. Credit... Raul Rubiera/Fayetteville Observer, via Associated Press

“We’re going to organize in the basic industries of our unions: construction, hospitality, health care, retail, food production and manufacturing,” said Tom Woodruff, director of strategic organizing for Change to Win, a federation of seven unions that includes the Service Employees International Union, the Teamsters and the United Food and Commercial Workers. “Those are jobs that are going to stay in the country. The question is whether those jobs are going to be decent middle-class jobs.”

Mark McKinnon, a media adviser to the presidential campaigns of John McCain and George W. Bush, is a spokesman for the Workforce Fairness Institute. Mr. McKinnon said the institute was focusing on drumming up grass-roots support from business. He would not say which companies are financing the institute, founded by several longtime Republican operatives.

“This issue has really become very high on the radar screen,” he said. “Businesses are hearing about it, and they are ready to riot in the street about it.”