He joined the Democratic party faction led by Pierre Soulé. In the Presidential Election of 1860, Hahn supported Stephen Douglas. He was fluent in English, French and German. In 1860 Hahn opposed secession, delivering a pro-Union speech in Lafayette Square. He avoided taking an oath of allegiance to the Confederacy. Opposed to secession and a supporter of the Union, Hahn was elected in 1862 as a Republican and the U.S. Representative from Louisiana's 2nd congressional district. This incorporated most of New Orleans, which had been occupied by Union forces. Hahn was one of two Louisiana Representatives seated in the 37th Congress, which adjourned on March 4, 1863, during the Civil War. Eventually, Hahn advised that there should be no more representation from Louisiana until it was "reconstructed." During his time in Washington, Hahn met and befriended President Abraham Lincoln. In 1864, with almost all of Louisiana under federal occupation, General Nathaniel P. Banks, the Union Military Commander of the Department of the Gulf (responsible, among other things, for civil order in occupied Louisiana), called state elections and convened a constitutional convention. Benjamin Franklin Flanders and Thomas Jefferson Durant, prominent Unionists, opposed the moderate plan called for by General Banks. Hahn purchased a pro-slavery newspaper, the "New Orleans True Delta," and used it to promote moderate Unionism supporting Banks' plan, including emancipation of slaves. Hahn ran for Governor for the Free-State Party and won the election with 54% or 11,411 votes. Fellows, a conservative Democrat, received 26% or 2,996 votes; and Benjamin Franklin Flanders, the radical Republican, received 20% or 2,232 votes. Hahn was elected as the first German-born governor of an American state. He is also claimed as the first ethnic Jewish governor in the United States; by then he was worshipping as an Episcopalian. On March 4, 1864, Hahn was inaugurated as Governor of Union-held Louisiana in an elaborate ceremony paid for by General Banks. As governor, Hahn supported universal education. In his term, Hahn tried to gain suffrage for freedmen and previously free people of color, but it was too early. He approved the state's ratification of the 15th Amendment. Hahn's administration made serious attempts to ensure enfranchisement of black Louisianans, laid the foundation for a public school system for blacks, and began an aborted Reconstruction in Louisiana.