The Republican party's organization is dated to 1856 by three passages, while a fourth gives 1854 (both readings reported).

The evidence

• “Mr. Fifield was a Republican in political sentiment from the time of the organization of that party in 1856, and though never an office seeker held several local offices. as member of the school board, alderman, etc. In October, 1853, Mr.”

  1908-brown-rock-county/34b-biographical-2.md, Vol. 2, PDF p. 386 / book p. 880

  Brown, William Fiske, editor. Rock County, Wisconsin: A New History of Its Cities, Villages, Towns, Citizens and Varied Interests, from the Earliest Times, Up to Date. Chicago: C. F. Cooper & Co., 1908. 2 vols. archive.org/details/rockcountywiscon01brow, archive.org/details/rockcountywiscon02brow.

  Grade A: Found on a page of one of our sources

  

• “Since the organization of the party, in 1854, the Republicans have invariably made a clean sweep of the county officers and State Senators. In 1856, Fremont's majority was 2,743 ;”

  1879-history-of-rock-county/12-rock-county-ch06-politics-war-record.md, PDF p. 441 / book p. 445

  Butterfield, C. W., et al. The History of Rock County, Wisconsin: its early settlement, growth, development, resources, etc. Chicago: Western Historical Company, 1879. Cornell University Library via the Internet Archive, archive.org/details/cu31924028871585.

  Grade A: Found on a page of one of our sources