Buchanan denied him the ambassadorship to Madrid in the Kingdom of Spain after the presidential election of 1856, thanks to the Ostend Manifesto. As a delegate to the pivotal, but soon violently-split 1860 Democratic National Convention in Charleston, South Carolina, Belmont supported influential U.S. Senator Stephen A. Douglas of Illinois, (who had triumphed in the famous 1858 Lincoln-Douglas Debates over his long-time political rival, the newly recruited Republican candidate Abraham Lincoln, in their battle for Douglas's Senate seat). Chairman of the Democratic National Committee Senator Douglas subsequently nominated Belmont as chairman of the Democratic National Committee. Belmont is attributed with single-handedly transforming the position of party chairman from a previously honorary office to one of great political and electoral importance, creating the modern American political party's national organization. He energetically supported the Union cause during the Civil War as a "War Democrat" (similar to former Tennessee Senator Andrew Johnson, later installed as war governor of the Union Army-occupied seceded state), conspicuously helping U.S. Representative from Missouri Francis P. Blair raise and equip the Union Army's first predominantly German-American regiment. Belmont also used his influence with European business and political leaders to support the Union cause in the Civil War, trying to dissuade the Rothschilds and other French bankers from lending funds or credit for military purchases to the Confederacy and meeting personally in London with the British prime minister, Lord Palmerston, and members of Emperor Napoleon III's French Imperial Government in Paris. He helped organize the Democratic Vigilant Association, which sought to promote unity by promising Southerners that New York businessmen would protect the rights of the South and keep free-soil members out of office. Post-war political career Remaining chairman of the Democratic National Committee after the War, Belmont presided over what he called "the most disastrous epoch in the annals of the Democratic Party". As early as 1862, Belmont and Samuel Tilden bought stock in the New York World in order to mold it into a major Democratic press organ with the help of Manton M. Marble, its editor-in-chief. According to the Chicago Tribune in 1864, Belmont was buying up Southern bonds on behalf of the Rothschilds as their agent in New York because he backed the Southern cause. Seeking to capitalize on divisions in the Republican Party at the War's end, Belmont organized new party gatherings and promoted Salmon Chase (the former United States Secretary of the Treasury