[5] He was elected to the Ohio State Senate in 1819 and served until 1821, having lost the election for Ohio governor in 1820.[33] He ran in the 1822 election for the United States House of Representatives, but lost to James W. Gazlay.[5][75] He was elected to the U.S. Senate in 1824, and was an Ohio presidential elector in 1820 for James Monroe[76] and for Henry Clay in 1824.[77]
Harrison was appointed in 1828 as minister plenipotentiary to Gran Colombia, so he resigned from Congress and served in his new post until March 8, 1829.[78] He arrived in Bogotá on December 22, 1828, and found the condition of Colombia saddening. He reported to the Secretary of State that the country was on the edge of anarchy, and that Simón Bolívar was about to become a military dictator.[78] He wrote a letter of polite rebuke to Bolívar, stating that "the strongest of all governments is that which is most free" and calling on Bolívar to encourage the development of democracy. 