Named after a section of the US Tax code, 527 organisations are political campaign groups officially unaffiliated to individual parties or candidates, and therefore not subject to campaign spending restrictions.

The groups have gained in prominence since 2002, when the McCain-Feingold campaign finance reforms were passed, cracking down on the use of "soft money" in election campaigns.

Critics of 527 groups say they are little more than front organisations allowing official campaigns to run expensive attack adverts without having to adhere to campaign finance restrictions.