From about the 1840s to the 1920s.
Source courtesy of this thread

Antebellum statesmen avoided the term altogether, preferring "the Union" or "the Republic". It was popularly believed that the shift was chiefly a result of the Civil War, where the grammatical debate parallels the political division of the time, though it seems the shift occurred independently of the war. The idea was promoted by Basil Lanneau Gildersleeve, a classics scholar and former Confederate soldier, though the shift and its connection to the war was noticed as early as 1887. There was debate for decades over the matter, based on the Constitution's use of the term as plural and its political implications.
Source