WASHINGTON—Saying it would eliminate the right to union representation for more than 600,000 workers, the International Brotherhood of Electrical Workers is urging members to tell Senate Democrats to filibuster the Tribal Sovereignty Act. The bill, passed by the House Jan. 10, would exempt any business or corporation owned by a Native American tribe from the National Labor Relations Act’s collective-bargaining protections. Supporters say it would prevent federal interference in tribal affairs. “Any legislation that takes away workers’ rights to organize—tribal sovereignty or not—is ridiculous,” IBEW Local 47 business manager Patrick Lavin said in a message to the union’s members. “We need IBEW members in every state to let their senators know we won’t stand for it.” Local 47 represents about 30 slot-machine technicians at the Morongo Casino and Resort in Cabazon, Calif., but last December, its owners, the Morongo Band of Mission Indians, refused to agree to a first contract. The California Labor Federation then launched a boycott of the casino. The Democrats on the fence include Heidi Heitkamp of North Dakota, Jon Tester of Montana, and Martin Heinrich of New Mexico, all of whom are running for re-election in states with large Native American populations.