Why did Justice Stevens disagree with the majority’s view of corporate political speech?

In his dissent in Citizens United v. Federal Election Commission, Justice John Paul Stevens argued that the majority’s decision threatened the integrity of elected institutions by disregarding stare decisis and misinterpreting the state interests behind campaign finance regulations.1

Justice Stevens disagreed with the majority’s view on several key points:

• Antidistortion Interest: He contended that the majority overgeneralized the rejection of identity-based restrictions. He argued that corporate domination of political speech during elections could impoverish, rather than enrich, the "marketplace of ideas".1
• Corruption: He criticized the majority’s definition of corruption as "simplistic and naive," arguing that limiting the concept to quid pro quo exchanges ignored broader risks to democratic integrity.1
• Corporate Democracy: He rejected the majority's belief that shareholders are adequately protected by internal corporate mechanisms, noting that the court greatly overestimated the ability of shareholders to hold corporate officers accountable through voting or derivative suits.1

Ultimately, Stevens warned that the ruling would "cripple" the ability of citizens and government bodies to protect the electoral process from corporate domination.1