The discovery of multiple explosive devices addressed to prominent Democrats across the U.S. today has the potential to be the kind “October surprise” that could upend the midterm elections and perhaps even affect the balance of power in Congress.

William Casey, campaign manager for Ronald Reagan’s 1980 presidential campaign, is said to have coined the term in the 1980s. But the concept of an 11th-hour revelation or event that could scramble everything in the month before Americans go to the polls predates the term.