"to meet or confer in caucus," 1850, from caucus (n.), but caucusing is attested from 1788.
"private meeting of party leaders or local voters," 1763, American English (New England), perhaps from an Algonquian word caucauasu "counselor, elder, adviser" in the dialect of Virginia, or from the Caucus Club of Boston, a 1760s social and political club whose name possibly derived from Modern Greek kaukos "drinking cup." Another old guess is caulker's (meeting) [Pickering, 1816], but OED and Century Dictionary find this dismissable.
CAUCUS. This noun is used throughout the United States, as a cant term for those meetings, which are held by the different political parties, for the purpose of agreeing upon candidates for office, or concerting any measure, which they intend to carry at the subsequent public, or town meetings. [John Pickering, "A Vocabulary, or Collection of Words and Phrases Which Have Been Supposed to be Peculiar to the United States of America," Boston, 1816]
The word caucus, and its derivative caucusing, are often used in Boston. The last answers much to what we stile parliamenteering or electioneering. All my repeated applications to different gentlemen have not furnished me with a satisfactory account of the origin of caucus. It seems to mean, a number of persons, whether more or less, met together to consult upon adopting and prosecuting some scheme of policy, for carrying a favorite point. [William Gordon, "History, Rise, Progress, and Establishment of the Independence of the United States of America," London, 1788]
1760, "that which stands first or highest in rank or importance;" see primary (adj.). Meaning "meeting of voters of the same political party in an election district for nominating candidates for office or delegates to a convention" is by 1855, American English, short for primary meeting (1823), primary election (1792, with reference to France; in a U.S. context from 1835), or primary caucus (1817). The system is informal, not constitutional, and has evolved over time.
Theoretically every voter belonging to the party in a district has a right to attend the primary and vote, but in cities and large places only registered voters who have answered certain test questions relating to party adherence have that privilege. [Century Dictionary, 1895]