David Costello, 65, is an Old Town native with over 30 years of government experience. He previously ran for independent Senator Angus King's seat in 2024, receiving 11% of the vote. Costello served as senior aide to Maryland's governor (2007-2011), led the Maryland Department of the Environment (2011-2015), worked for the U.S. Agency for International Development (USAID) in conflict-torn regions, and served as interim climate and clean energy program director for Maine's Natural Resources Council. His 2026 campaign emphasizes substantive reform agenda and governance experience, positioning himself between Mills and Platner as a serious policy-focused candidate without their controversies.
Born in Bangor and raised in Old Town, Maine by his mother and maternal grandparents. His father was an Army veteran and labor organizer, and his grandfather was French and Irish American. Costello earned a bachelor's degree from George Washington University (1983) and a graduate degree from the London School of Economics (1989). He has worked as an environmental policy consultant and held senior government positions managing multi-million-dollar programs in the United States and abroad, particularly in conflict regions including Cambodia, Haiti, Bosnia, Croatia, Serbia, and Kosovo.