By Jody Godoy and Ross Kerber

May 22 (Reuters) - U.S. federal antitrust enforcers were expected to express support on Thursday for arguments wielded by Republican states that accuse asset managers BlackRock BLK.N, Vanguard and State Street STT.N of conspiring through climate activism to decrease coal output, two sources familiar with the matter told Reuters.

The U.S. Department of Justice and Federal Trade Commission were expected to file a statement of interest in the case where Texas and 12 other states claim the companies used their substantial holdings in U.S. coal companies to discourage competition.

The development marks a political setback for the top asset managers. With some $27 trillion among them, BlackRock, Vanguard and State Street have come under fire from conservative Republicans, many from energy-producing states, who say the firms wrongly put environmental and social concerns above maximizing returns for their customers.

(Reporting by Jody Godoy and Ross Kerber; editing by Chris Sanders and Matthew Lewis)

((Jody.Godoy@thomsonreuters.com;))