A reference like this is called an allusion. Why do you think Abraham Lincoln alludes to a founding document in his speech?
Abraham Lincoln may have alluded to a founding document, such as the Declaration of Independence, in his speech to connect his message to the principles and values of the country's founding fathers. By referencing a founding document, Lincoln may be trying to emphasize the importance of upholding the ideals of equality, freedom, and justice that are embodied in such documents. Additionally, alluding to a founding document can add depth and credibility to Lincoln's message, as it shows that his ideas are rooted in the foundational principles of the United States.
Furthermore, by alluding to a founding document, Lincoln may be appealing to the sense of national identity and patriotism among his audience, encouraging them to reflect on the nation's history and values in order to inspire unity and inspire them to work towards a common goal.