He was not originally a republican (they were abolitionists), he joined the party because it gave him a better chance to win.
Republicans were not abolitionists, except for a few radicals. The Republicans in the 1850s were always a Free Soil party. Among other things their biggest gripes were the Fugitive Slave Act, Kansas-Nebraska Act, and Dredd Scott. All issues related to slavery in the North and/or territories.
The Liberty Party were the abolitionists.
Abolitionists were anti-slavery people, but not all anti-slavery people were abolitionists.