originally presidential candidates were selected by party caucuses in congress
The system was introduced after George Washington had announced his retirement upon the end of his second term, when the Democratic-Republican Party, and Federalist Party began contesting elections on a partisan basis. Both parties may have held informal caucuses in 1796 to try to decide on their candidates. After the disorganized electoral voting of 1796, both parties held formal caucuses in 1800 that selected their respective presidential candidates (prior to the ratification of the 12th Amendment in 1804 each party ran two presidential candidates). The Federalists secretly held their caucus in early May, but later made their ticket of President John Adams and Charles Cotesworth Pinckney public. The Democratic-Republicans also secretly met in May, and agreed to equally support Thomas Jefferson and Aaron Burr for president. The Federalists did not hold another caucus after 1800.
Congressional nominating caucus