Final answer:
The Republican Party was organized largely due to disagreements over the issue of slavery and its expansion into new territories and states, culminating in the Civil War era.
Step-by-step explanation:
The organization of the United States Republican Party resulted largely from disagreements over the issue of slavery and its expansion into new territories and states. The first signs of the creation of what would become the Republican Party can be traced back to the debates surrounding the admission of the State of Missouri in 1821, which highlighted the deep divide over slavery. As the country expanded westward and acquired new territories, such as from the war with Mexico, the debates and divisions grew stronger.
This led to the formation of an antislavery coalition in the mid-1850s that called itself the Republican Party, which was committed to preventing the spread of slavery to areas where it did not already exist. The election of the first Republican president, Abraham Lincoln, in 1860, solidified the party's significance and set the nation on the path to the Civil War.