During his career, Thomas Nast used the power of the cartoon to challenge issues such as slavery, segregation, the inflation of the currency and the Ku Klux Klan and was instrumental in the presidential election of Ulysses Grant in 1868 and 1872; in the latter campaign, Nast's ridicule of Horace Greeley's candidacy was especially merciless. Nast became a close friend of President Grant and the two families shared regular dinners until Grant's death.
His cartoons were, however, at their most 'poisonous' and effective in the downfall of Boss Tweed, who so feared Nast's campaign that an emissary was sent to offer Nast a $500,000 bribe (a huge sum at that time) to "drop this Ring business" and take a trip abroad. Declining the offer, Nast pressed his attack, and Tweed was arrested in 1873 and convicted of fraud.
Many cartoons were produced by Nast in this campaign to rid the City of the embezzlement and corruption rife within the Tweed City Hall administration including one which depicted all the main characters standing in a circle, each pointing the finger of blame at another.
With the death of Fletcher Harper in 1877, Nast lost an important champion at Harper's Weekly Journal, and his contributions became less frequent. He focused on oil paintings and book illustrations, but these are comparatively unimportant. He quit Harper's Weekly in 1886. Nast lost his forum and in losing him, Harper's Weekly lost its political importance.