Robert Drew's 1960 fly-on-the-wall of the JFK-Humphrey Wisconsin primary. The birth of direct cinema and modern political documentary.
From Wikipedia
Primary elections or primaries are elections held to determine which candidates will run in an upcoming general election. In a partisan primary, a political party selects a candidate. Depending on the state or party, there may be an "open primary", in which all voters are eligible to participate, or a "closed primary", in which only members of a political party can vote. Less common are nonpartisan primaries in which all candidates run regardless of party.