An attorney for BlackRock called antitrust claims by Republican-led states "unprecedented, unsound and unsupported" on Monday and said they had failed to show how the firms' involvement with industry climate groups interfered with market competition.

Gibson Dunn attorney Gregg Costa spoke as BlackRock and co-defendants Vanguard and State Street seek to dismiss the claims in the closely watched antitrust case brought by Texas and 12 other states.

The outcome could have major implications for how the companies, which together manage some $27 trillion, manage their holdings and passive funds.

The states allege the fund firms' climate activism pressured coal companies to slash output and drive up utility bills. The companies, which continue to own fossil fuel stocks, say their proxy voting hardly changed and there is no evidence they leaned on companies to cut production.

Speaking for the states, Cooper & Kirk attorney Brian Barnes said even calling attention to environmental matters could have an impact.

"Jawboning by these defendants as to decisions about market strategy just very clearly has the potential to influence output decisions at the coal company," Barnes said.