By Robert Iafolla

WASHINGTON (Reuters) - The U.S. Supreme Court on Monday declined to wade into a legal battle pitting Native American tribal sovereignty against the federal government's power to regulate labor relations in cases involving casinos on Indian land in Michigan.

In deciding to stay out of that fight, the court let stand a pair of federal appeals court rulings that gave the U.S. National Labor Relations Board authority over casinos on Indian land.

Those rulings boosted unions' ability to organize workers in the tribal casino industry, which generated $28.5 billion in gaming revenue in 2014, the most recent year for which figures were available, according to the National Indian Gaming Commission.

With the Supreme Court declining to take up the issue, the Republican-led Congress stands as the tribes' best hope of avoiding NLRB jurisdiction. The U.S. House of Representatives passed a bill in November to strip the labor board's authority over tribal businesses on Indian lands, but that has been stalled in the Senate. That means the tribes must abide by the two decisions from the Cincinnati, Ohio-based 6th U.S. Circuit Court of Appeals.

The NLRB has said it can intervene in a tribe's labor practices when the tribal business is commercial rather than governmental in nature and both employs and caters to non-Native Americans. The tribes have argued that federal labor law does not give the labor board the power to cast aside tribal sovereignty and assert jurisdiction over their businesses.

In 2015, the 6th Circuit upheld an NLRB decision requiring the Little River Band of Ottawa Indians to stop enforcing a tribal law that restricted union powers at its casino in Michigan. Three weeks later, the appeals court ruled that the NLRB could order a casino owned and operated by the Saginaw Chippewa Indian Tribe of Michigan to reinstate a housekeeper who was fired for soliciting union support.

The two tribes said in their petitions for Supreme Court review that other U.S. appeals courts are split on the federal government's jurisdiction over tribal businesses on Indian land.

The Saginaw Chippewa Indian Tribe said in its brief that "the law in this area is - to put it charitably - a mess."

The cases are Little River Band of Ottawa Indians v. NLRB, No. 15-1024, and Soaring Eagle Casino and Resort v. NLRB, No. 15-1034, in the U.S. Supreme Court.

(Additional reporting by Daniel Wiessner)