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.
Costello argues fixing democracy itself is prerequisite for addressing other issues
Unlike Mills (who emphasizes experience) and Platner (who emphasizes economics), Costello's entire platform rests on institutional reform: Electoral College elimination, campaign finance overhaul, House expansion, Senate restructuring, Supreme Court term limits, and making amending the Constitution easier.
Voting History: N/A - No voting record, government experience only
Costello remains largely unknown in Maine despite significant government experience
Polling shows Costello consistently in single digits (4% in recent polls), far behind Mills and Platner. This reflects limited visibility despite his substantive background.
Voting History: N/A
Costello's campaign has been notably free of scandal or controversy
Unlike Platner, Costello has not faced attacks over past statements or behavior. Unlike Mills, he hasn't faced questions about age or establishment backing. His main weakness is obscurity, not controversy.
Voting History: N/A
Costello has no Senate voting record. His government experience was in executive and administrative capacities, not legislative positions. He has no voting history to evaluate.
Costello's social media maintains a serious, policy-focused tone emphasizing governance expertise and institutional reform. His communication is less activist or populist than Platner, and less biographical than Mills. He focuses on substantive issues rather than personal story or movement-building.
Posts about Democratic reform agenda, nuclear arms control, immigration policy, and government dysfunction
Costello's social media emphasizes substantive policy discussion over political theater or grassroots activism
Detailed policy platform and reform proposals for Democratic institutions
Costello directs voters to his website for detailed policy information rather than building social media presence
Mills is more electable due to name recognition and electoral track record
Mills argues she has proven ability to win statewide elections, implicitly suggesting lesser-known candidates like Costello are less viable.
Platner's populist agenda is more compelling than establishment-style governance reform
Platner's grassroots movement contrasts with Costello's technocratic governance approach, positioning Costello as too establishment-focused despite his policy focus.