The prosperous owner of a Cleveland coal and iron enterprise, Hanna soon expanded his interests to include banking, transportation, and publishing. Convinced that the welfare of business (and consequently the prosperity of the nation) was dependent upon the success of the Republican Party, he began as early as 1880 to work among industrialists to ensure the financial support of likely candidates for office. He was especially impressed by Ohio congressman William McKinley's (McKinley, William) successful sponsorship in 1890 of a high protective tariff, and thenceforth he devoted all his energies to McKinley's political advancement, first as governor (1892–96) and then as president (1897–1901). In preparation for the 1896 contest with the Democrat–Populist candidate, William Jennings Bryan (Bryan, William Jennings), Hanna was reputed to have poured more than $100,000 of his own money into preconvention expenses alone. Raising an unprecedented fund from wealthy individuals and corporations, the dynamic Hanna skillfully directed the $3,500,000 campaign—the costliest and best organized the nation had ever witnessed. At a rate of spending exceeding his opponents by 20 to 1, his 1,400 paid workers inundated the country with millions of pamphlets promising continuing prosperity with McKinley. Hanna succeeded in stunting Bryan's grass-roots appeal with a continual barrage of posters and propaganda that preceded and followed Bryan at every whistle-stop of his campaign train.

Once in office, McKinley helped to fulfill Hanna's lifelong ambition by appointing Sen. John Sherman secretary of state, thus creating a vacancy in the U.S. Senate. Hanna was elected to fill the vacancy (March 1897) and remained in the Senate until his death.