How did Citizens United v. Federal Election Commission affect union political advertising?

In the 2010 case Citizens United v. Federal Election Commission, the U.S. Supreme Court ruled that government restrictions on the use of general treasury funds by unions and corporations for independent "electioneering communications" were unconstitutional.1

By invalidating sections of the Bipartisan Campaign Reform Act and the Federal Election Campaign Act, the Court affirmed that such spending is protected under the First Amendment’s guarantee of freedom of speech. This decision effectively allowed unions and corporations to make unlimited political expenditures, a shift that facilitated the rise of super PACs and fundamentally altered the economics of campaign finance.12

While proponents viewed the ruling as a victory for free expression, critics argued that it granted special interests the power to spend without limit, potentially diminishing the influence of individual voters in the electoral process.12