One of the major themes present in The Radical and the Republican is the Constitution of the United States and how it governs the role of the American government. As the supreme law of the land, the Constitution dictates how the government must behave in all situations, and it both limits and extends certain powers to individual branches so that no singular entity can become too strong.

The problem regarding the Constitution is that it is not a clear document. It must be interpreted by each generation, which can often lead to confusion over the meaning and intent of the framers and Founding Fathers. For The Radical and the Republican, the theme of the Constitution is returned to over and over again because of how differing parties, particularly Lincoln and Douglass, view the power of the federal government in ending slavery.

As a strong supporter of the Constitution, Lincoln believed that it was an anti-slavery document. However, he struggled with how much power and what powers, exactly, it extended to him as President of the United States. He initially favored acting through Congress, so as to give the perception that the end of slavery was a popular opinion among the people, not a single, authoritarian act on the part of an anti-slavery president.