Historian Lewis E. Lehrman points out that President Lincoln asserted the precept of equality perhaps most in his Gettysburg Address when he emphasized that the founders brought forth upon this continent a new nation in 1776. He chose the year of the Declaration of Independence was signed rather than 1788, the year the Constitution was ratified.
Lehrman says Lincoln’s rhetorical and political strategy was necessary because a nation founded upon the proposition of human equality must have been, at its inception, antislavery in principle.
He points out that Lincoln also grounded his lengthy antislavery speech at Peoria on Oct. 16, 1854 on the Declaration: “Let us re-adopt the Declaration of Independence, and, with it, the practices, and policy, which harmonize with it. Let north and south — let all Americans — let all lovers of liberty everywhere — join in the great and good work.”
Source: Lincoln at Peoria: The Turning Point by Lewis E. Lehrman