Final answer:
Northerners continued to support the Democratic Party due to economic concerns over preserving Southern trade, the party's focus on uniting Americans on other issues than slavery, and the perception of the Democrats being committed to preserving the Union (option E).
Step-by-step explanation:
Many northerners were still willing to vote Democratic instead of Republican as late as 1856 because of several key factors. One significant reason is related to the concern for maintaining profitable business connections with the South. Numerous Democrats involved in interstate commerce did not want to jeopardize their economic interests, which were tightly linked to Southern trade. Additionally, the Democratic Party, appealing to a broad range of voters including small farmers, urban workers, and immigrants, managed to navigate the contentious issue of slavery by focusing on uniting Americans around other issues such as white supremacy and expansionism.
Lastly, with the onset of the Civil War and the division within the Democratic Party itself, many saw the Democrats as the party dedicated to preserving the Union at all costs. This multiplicity of reasons shows why the Democratic party maintained a degree of support in the North, despite the emerging Republican challenge.