Final answer:
The Democratic Party's struggle in the 1928 presidential election was best exemplified by its divided stance on Prohibition, reflecting deep conflicts between urban "wets" and rural "dries" which hampered the party's electoral success.
Step-by-step explanation:
The Democratic Party faced significant internal conflict during the 1928 presidential election due to a fundamental contradiction exemplified by their need to cater to diametrically opposed constituencies: urban, northern "wets" who opposed Prohibition and rural, southern "dries" who were in favor of it. This tension within the party undermined their unity and opened the door for Republican ascendancy during the era. Moreover, while the party attempted to avoid the contentious issue of slavery, it struggled to maintain harmony between diverse voter bases, particularly as different regions and social groups had conflicting interests and priorities.
In the context of the options provided and the historical setting, the most accurate contradiction that exemplifies the Democratic Party's struggle during the 1928 presidential election is option C - the party was forced to publicly endorse Prohibition despite the objections of big businesses and urban constituents.