Final answer:
The political activity of Citizens United during the 2008 primary election was to air a documentary critical of Hillary Clinton, which led to a Supreme Court case that struck down limits on corporate and union electioneering communications, significantly altering campaign finance law.
Step-by-step explanation:
The political activity that the group Citizens United engaged in during the 2008 primary election, which led to the Supreme Court case Citizens United v. Federal Election Commission, was their attempt to advertise a documentary critical of then-presidential candidate Hillary Clinton. This action was claimed to violate the Bipartisan Campaign Reform Act (BCRA) because it was a campaign communication that occurred close to an election, and was funded by a corporation. The case escalated to the Supreme Court, which ruled in a 5-4 decision that the restrictions on corporate and union expenditures during election cycles, as set forth by the BCRA, were in violation of the First Amendment. This ruling affirmed that corporations and labor unions could spend unlimited sums in support of or opposition to candidates, as long as the spending was independent of the campaigns.
The Citizens United decision has led to significant changes in campaign finance, including an increase in election spending and the rise of Super PACs, which can now accept and spend unlimited funds provided their activities remain independent of any candidate's campaign. This case remains a pivotal moment in the history of political finance in the United States.