A few , mostly Western , states adopted primary elections in the late 19th century and during the Progressive Era , but the catalyst for their widespread adoption came during the election of 1968 . The Vietnam War energized a large number of supporters of anti-war Senator Eugene McCarthy of Minnesota , but they had no say in the matter . Vice President Hubert Humphrey — associated with the unpopular administration of Lyndon B. Johnson — did not compete in a single primary , yet controlled enough delegates to secure the Democratic nomination . This proved one of several factors behind rioting which broke out at the Democratic convention in Chicago
Democratic convention in Chicago
1968 Democratic National Convention
United States presidential nominating convention