Upon his return, Weaver worked briefly as a store clerk before resuming the study of law. He enrolled at the Cincinnati Law School in 1855, where he studied under Bellamy Storer. While in Cincinnati, Weaver began to question his support for the institution of slavery, a change biographers attribute to Storer's influence. After graduating in 1856, Weaver returned to Bloomfield and was admitted to the Iowa bar. By 1857, he broke with the Democratic party of his father to join the growing coalition that opposed the expansion of slavery, which became the Republican Party.