Weeks after passing resolutions opposing Kinder Morgan’s Permian Highway Pipeline, Kyle and Hays County Tuesday formally became plaintiffs in a multi-party lawsuit against the Houston-based energy firm.

Kyle city leaders tossed their hats into the fray when they joined the lawsuit via a 6-0 vote Tuesday. Earlier in the day, Hays County Commissioners voted 5-0 to become a party in the litigation.

Additional plaintiffs in the suit, expected to be filed in th...