Congress soon may make it more difficult for labor unions to organize workers at tribal casinos by shrinking the power of the National Labor Relations Board, the main federal labor law enforcement agency.

The Tribal Labor Sovereignty Act would amend the National Labor Relations Act to explicitly state that the NLRB's enforcement powers don't extend to Native American reservations, reversing a decade-old board decision. The issue cuts across party lines because some tribes have strong pull in their home states, as does the gambling industry.

The bill passed the House last week on a bipartisan 239-173 vote with 23 Democrats joining 216 Republicans to support it. The legislation was subsequently attached to a Senate bill sponsored by Sen. Jeff Flake, R-Ariz., that passed by voice vote last year.

Flake's bill didn't include the NLRB language, however, so Senate lawmakers will have to vote on it a second time to reconcile it with the House version — and this time pro-labor Democrats are expected to make it a battle. "Yes, it will likely be a tough vote this time around," said a Senate Republican source who requested anonymity. "We want to see this sent to the president as soon as possible."

Hill sources expect that the legislation will be taken up in mid-February, when the National Congress of American Indians, the largest tribal federation, holds its annual convention in Washington. The tribes are expected to use the opportunity to heavily lobby Congress.

Democrats expect significant defections. "There are plenty of Senate Democrats who will support this... There are members who are very divided. It forces Democrats to choose between two constituencies: unions and the tribes," said a Democratic congressional source who requested anonymity.

Tying the issue to Flake's legislation, which originally dealt with tribal water rights, is expected to bring along a few additional votes since it is an important local issue for some tribes and their Senate allies. Democratic aides believe the Senate GOP leadership held onto the original Flake legislation, which passed almost a year ago, rather than forwarding it to the House as stand-alone bill solely to boost the odds for the NLRB rollback effort. According to one congressional source, the Senate GOP leadership will be able to limit debate on the Flake legislation because of the previous vote.

The National Labor Relations Act, which passed in 1935, is the main law governing workers' right to form unions. The law does not specifically address tribal lands, however, and by tradition, the board took a hands-off approach to reservations. That changed in 2004 when the board, then led by George W. Bush appointees, asserted that it did have the authority and would start to enforce it.

The NLRB's move was a big win for unions. Labor leaders have long sought to organize tribal casino workers, most of whom are not tribe members. Some tribes, such as the Navajos, have allowed unions on their lands, but labor groups and tribal leaders have clashed elsewhere. Most tribes have lobbied Congress to overturn the NLRB's move, arguing that being forced to abide by federal regulations will undermine the profitability of their casinos, the main sources of revenue for tribes.

Rep. Virginia Foxx, R-N.C., chairwoman of the House Education and the Workforce Committee, and Rep. Tim Walberg, R-Mich., who heads the labor and pensions subcommittee, called the Tribal Labor Sovereignty Act a "long-overdue solution to protecting the rights of Native Americans, and respecting their laws the same as state and local governments."

Democrats argue it's little more than an attempt to hobble unions and denies casino employees workplace protections. "There is no principled basis for excluding hundreds of thousands of workers from coverage under our nation’s labor laws just because they work in commercial enterprises on tribal lands," said Rep. Bobby Scott of Virginia, the committee's top Democrat.