Final answer:
The Supreme Court ruling in Citizens United v. Federal Election Commission allowed unlimited campaign spending by corporations and unions based on the protection of free speech under the First Amendment. This decision overturned previous restrictions on campaign spending and resulted in a significant increase in election spending.
Step-by-step explanation:
The Supreme Court ruling in Citizens United v. Federal Election Commission had the effect of allowing unlimited campaign spending by corporations and unions. The Court determined that campaign expenditures are a form of speech protected by the First Amendment, and therefore the restrictions imposed by the Bipartisan Campaign Reform Act (BCRA) violated the corporations' right to free expression. This decision overturned previous restrictions on campaign spending and led to a significant increase in election spending.