Here in the New York Times:

The threat that labor poses to the bill has gone largely unrecognized in part because three prominent unions — the service employees, the farm workers, and the hotel, restaurant and apparel workers — have backed the legislation. But that support, advocates say, has been outweighed by opposition from the A.F.L.-C.I.O. and virtually all other unions, including auto workers, Teamsters, food and commercial workers, and construction unions.