The renewal of the slavery issue in the mid-1850s led Bigelow, [William Cullen] Bryant, and the Evening Post to cast their lot with the new Republican party. In 1856 Bigelow wrote a campaign biography for John C. Frémont, the party's first presidential nominee. As a reward for his constant labors for the Post, Bigelow began an eighteen-month European vacation in 1858 that provided him with contacts among liberal intellectuals and politicians who proved valuable to his later diplomatic career.