Before the 20th century , only the party leaders in each state could nominate presidential candidates. At a caucus, local party members gather to nominate a candidate. A caucus is a lively event at which party leaders and activists debate issues, consider candidates, choose delegates, and discuss the party platform, or statement of principles. The rules governing caucus procedures vary by party and by state. A primary is more like a general election.
Primary elections are the main way for voters to choose a nominee.
Nominee for president is announced at national party conventions. The main goal of a national party convention is to unify party members behind the party's platform and nominees. Thousands of delegates gather to rally support for the platform and to nominate candidates for president and vice-president. From the 1820s until the 1930s, party conventions were boisterous events in which determining a nominee could spark hot debate. By the mid-20th century, however, primary elections had become the main way of selecting a nominee.