The Mashpee Wampanoag, seeking to build a tribal casino in Taunton, are disputing the authority of the state gambling commission to license a commercial casino in Southeastern Massachusetts. The commission began a controversial debate last week on lifting a ban on commercial casinos in the southeast region. In a letter delivered to the commission Thursday, the Mashpee Wampanoag said that “the tribe continues to believe the commission lacks lawful authority” to solicit bids from commercial developers in the southeast region. The 2011 state casino law delays bidding on commercial casino development in the southeast region to give the tribe time to make progress on a casino proposal.Mark Arsenault can be reached at email@example.com. Follow him on Twitter @bostonglobemark.