Final answer:
The Republican Party, founded in the 1850s, endorsed a policy of opposing the spread of slavery to new territories, a stance initiated by Stephen Douglas with the Kansas-Nebraska Bill. They nominated John C. Frémont in 1856 under the slogan 'Free Soil, Free Speech, Free Men, Frémont!' the party's antislavery position eventually leading to the critical election of 1860.
Step-by-step explanation:
The Republican Party, which was founded in the 1850s, strongly endorsed a policy about slavery in the territories that Stephen Douglas had begun advocating in the late 1850s. This policy opposed the spread of slavery into the new territories.The Republican Party formed as a political coalition to stop the spread of slavery after Stephen Douglas introduced the Kansas-Nebraska Bill. The party was comprised of Democrats, Free Soilers, Whigs, and Know-Nothings united against the Democrats' control.
During the presidential campaign of 1856, the Republicans nominated John C. Frémont, opposing the expansion of slavery into the territories, which they articulated with the slogan "Free Soil, Free Speech, Free Men, Frémont!" Events throughout the late 1850s, including Lincoln's speeches and the division within the Democratic Party, helped the Republican Party gain momentum against the extension of slavery, setting the stage for the pivotal election of 1860.