After the war, Campbell was appointed by the legislature to a state circuit court judgeship. In 1851, he received the Whig nomination for governor. His opponent was William Trousdale, his old Seminole War superior, who had been elected in a close election in 1849. Trousdale had endorsed the Nashville Convention of 1850, in which southern states threatened to secede if the federal government attempted to ban slavery in newly acquired territories, and blasted the Compromise of 1850, which admitted California as a free state. He accused Campbell of being "too favorable to northern views." Campbell on the other hand called the Compromise a "work of wisdom," and derided the Nashville Convention as treasonous. Running with the campaign slogan, "Boys, follow me!" which he had yelled at the Battle of Monterrey, Campbell edged Trousdale by a vote of 63,333 to 61,673. As governor, Campbell called for a more rational approach to resolving sectional strife, and demanded an end to the "insane" talk of secession. After his initial term in 1853, he did not run for reelection, instead choosing to accept a position as President of the Bank of Middle Tennessee in Lebanon, Tennessee. The Whigs nominated Gustavus A. Henry, who was defeated by the Democratic candidate Andrew Johnson in the general election. Following the collapse of the Whig Party in the mid-1850s, Campbell threw his support behind the American Party ("Know Nothings"), which many Tennessee Whigs had joined. He was again appointed to a circuit court judgeship in 1859, and continued campaigning against secession. In 1860, he supported Constitutional Union Party candidate and fellow Tennessee Whig John Bell for president. In May 1861, as a wave of pro-secession sentiment swept through Middle Tennessee in the wake of the attack on Fort Sumter, Tennessee's ex-Whigs and others opposed to secession convened in Nashville, and nominated Campbell to oppose Isham Harris in the gubernatorial election later that year. Campbell declined this offer, however. Governor Harris offered him a command in the Confederate Army, but he declined this, as well. In early 1862, after the Union Army occupied Middle Tennessee, Assistant Secretary of War Thomas A. Scott recommended Campbell for military governor, but the appointment went to Andrew Johnson. On May 12, 1862, Campbell chaired a convention of Tennessee Unionists that mapped out a plan to bring the state back into the Union. Shortly afterward, President Abraham Lincoln appointed him brigadier general in the Union Army.