Short Answer 
The Republican Party was founded in the 1850s to oppose the expansion of slavery, culminating in a strong anti-slavery platform during the 1860 presidential election. Their commitment to preventing slavery’s spread resonated with voters and was rooted in the earlier efforts of anti-slavery politicians from the Opposition Party.
Step 1: Understand the Republican Party’s Position
The Republican Party emerged in the 1850s, primarily focused on opposing the expansion of slavery. Their platform during the 1860 presidential election emphasized preventing slavery from spreading into newly acquired territories, which was crucial in shaping the political landscape of the time. The party aimed to attract support from those who were against the institution of slavery as it was seen as morally and economically wrong.
Step 2: Examine the 1860 Presidential Election
During the 1860 presidential election, the Republican Party held a convention in Chicago where they adopted a clear stance against slavery’s expansion. The platform outlined their commitment to preventing the reopening of the slave trade and articulated a vision for a nation free from the spread of slavery into the West. This attracted many voters who were concerned about slavery’s impact on free labor and democracy.
Step 3: Recognize the Historical Context of Opposition
Prior to the formation of the Republican Party, several anti-slavery politicians operated under the Opposition Party, which included former members of the Democratic and Whig Parties. This group became active in the 1850s as a response to the increasing threat of slavery expanding into new territories. Their efforts laid the groundwork for the Republican Party’s stronger and more unified opposition to slavery at the time of the 1860 election.