Many historians consider Lincoln’s Cooper Union speech to have been a seminal step on his road to the presidency.
Lincoln had not yet declared himself a candidate for the presidency; rather, he was speaking as a “Republican Party Statesman.” The goal of his speech (aside from gaining national exposure) was to prove, in contrast to the allegations of Senator Stephen Douglas, that the Founding Fathers had indeed intended for slavery to die out, and not to be expanded into new territories.
On the afternoon of the speech, Lincoln strolled the streets of Manhattan and stopped off at the studio of Matthew Brady to have a photograph taken.
At Cooper Union, Lincoln went through the voting records of the Founders for the audience, demonstrating that the majority of those who signed the Constitution and the Bill of Rights supported the prevention of the spread of slavery into federal territories. Not only that, but these men had voted for the exclusion of slavery both before and after the formation of the Constitution. This argument was meant to vitiate the contention of both Douglas and Supreme Court Justice Taney that when a U.S. citizen entered a federal territory, “the Federal Government can exercise no power over his person or property, beyond what that instrument confers, nor lawfully deny any right which it has reserved.” (Justice Taney writing in the Dred Scott decision, in which he famously denied that negroes were citizens.) [Taney’s position was not only reflective of accepted U.S. law at the time, but may have been constitutionally correct. See, e.g., Dred Scott and the Problem of Constitutional Evil by Mark A. Graber.]
Lincoln also emphasized that the Republican Party did not seek to wipe out slavery where it was constitutionally permitted: “Wrong as we think slavery is, we can yet afford to let it alone where it is.” Rather, he said, Republicans merely fought its expansion. They were also opposed to the resumption of the slave trade (which Lincoln explained would be a natural outgrowth of a law giving people the right to procure slaves in the new territories), but supported the Fugitive Slave Law (because it was the legislated law of the land).
Lincoln ended with the inspiring call: “let us have faith that right makes might, and in that faith, let us, to the end, dare to do our duty as we understand it.”
The audience gave Lincoln a standing ovation. Lincoln, never short on ambition, went over to the New York Tribune offices after the speech. Although it was nearly midnight, he sat up and reviewed and corrected the galley proofs for his speech that would be printed in the next day’s newspaper. The speech, reproduced throughout the states, gave Lincoln great exposure, and garnered praise from both Republicans and Democrats alike. The New York Tribune hailed it as “one of the most happiest and most convincing political arguments ever made in this City … No man ever made such an impression on his first appeal to a New-York audience.”
You can read the entire text of Lincoln’s historic speech here.