The Metropolitan Transportation Authority is vowing to fight back against a judge's ruling that one of its funding streams is unconstitutional.

The MTA already faces mountains of debt because of paltry state and federal support.New York State Supreme Court Judge Bruce Cozzens on Wednesday said the mobility tax "does not serve a substantial state interest." He said that to pass constitutional muster, the law that created the funding stream would have required special legislation or a two-thirds supermajority vote in both houses of the state Legislature; the law was approved by a 60 percent majority vote in the Assembly and 52 percent majority vote in the state Senate back in 2009.

The MTA vowed to appeal Wednesday’s ruling, saying "We believe this opinion will be overturned, since four prior challenges to the constitutionality of the law making the same argument have been dismissed."

The MTA, which operates the nation's largest mass-transit system, faces  serious financial headwinds. Without proper investment from the state or federal governments, the transit authority has had to rely on borrowing and fare hikes to cover its costs.

Transit groups warned of serious service cuts and financial turmoil if revenue streams dry up. The mobility tax raises $1.26 billion annually for the MTA.