What are the most expensive midterm races?

The 2026 midterm elections are projected to be the most expensive in history, with Republicans investing heavily in battleground races to maintain control of Congress. AdImpact reports that political advertising spending exceeded $10 billion in October, reaching an all-time high. These elections serve as a referendum on Donald Trump's performance, as voters decide if Republicans will retain power or if Democrats will take control to stymie the president's agenda during his term.