A 2002 campaign finance reform law named after its main sponsors: Republican Senator John McCain of Arizona and Wisconsin Democrat Russ Feingold. Aspects of the law were overturned by the Supreme Court's Citizens United ruling in 2010.

The law is designed to limit the underground system of fundraising and spending in federal election campaigns. It bans "soft money" to national political parties and restricts "issue ads" benefiting candidates. These two practices became increasingly common in elections after the 1974 Buckley vs Valeo Supreme Court decision left loopholes in campaign disclosure laws and limits on contributions.