Read this excerpt from Abraham Lincoln’s Emancipation Proclamation. What was his purpose in including a reference to the US Constitution?
And by virtue of the power and for the purpose aforesaid, I do order and declare that all persons held as slaves within said designated States and parts of States are, and henceforward shall be, free; and that the Executive Government of the United States, including the military and naval authorities thereof, will recognize and maintain the freedom of said persons. And I hereby enjoin upon the people so declared to be free to abstain from all violence, unless in necessary self-defense; and I recommend to them that, in all case when allowed, they labor faithfully for reasonable wages. And I further declare and make known that such persons of suitable condition will be received into the armed service of the United States to garrison forts, positions, stations, and other places, and to man vessels of all sorts in said service. And upon this act, sincerely believed to be an act of justice, warranted by the Constitution upon military necessity, I invoke the considerate judgment of mankind and the gracious favor of Almighty God.

to embarrass the Confederate states
to show that he could be a fair and judicial leader
to portray himself as a great military leader
to establish that the position stated in the document is legal

The Emancipation Proclamation is a document that states that all slaves are to be free in the United States. This was the result of the Civil War between the Confederacy and the Union. In this document, Lincoln explains what the consequences of the proclamation are, and what the former slaves are to expect from it. It also references the US Constitution in order to show that the position stated in the document is legal.