Reddit mentions of Lincoln at Cooper Union: The Speech That Made Abraham Lincoln President (Simon & Schuster Lincoln Library)

I'm not sure where you got that. According to Harold Holzer (who literally wrote a book on the speech we're talking about):

A synopsis of the book by the Chicago Tribune:

>Finally, he rallied Republicans with a call to be true conservatives by holding firm to what he saw as the anti-slavery heritage of the American republic and standing strong against Southerners who were trying to expand human bondage. " '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,' " he admonished Republicans.

Here's the book:

https://www.amazon.com/Lincoln-Cooper-Union-President-Schuster/dp/0743299647

As I understand the speech, Lincoln was seen as a radical. This speech was written to establish himself and his party as conservative, to demonstrate such so that he could be elected.

Now, I think what's confusing the issue is that the act was progressive, right? No doubt slavery was an established part of US culture at the time of the speech. But the TONE of the speech is conservative, and Lincoln is making a very good argument that the increase in slavery in the US was in fact the "progressive" movement, and that going back to the original intention of the country was the truly conservative path.

Now, you can cry foul and say that Lincoln overstepped, but there are some problems:

1. you're disagreeing with Lincoln, and he was a pretty damn smart dude
   
2. he was re-elected, so apparently his message rang true for people of the time, it's hard to say what connections they had to the constitution since some states were rebuking it at the time.
   
   even if you are ok with those points, Lincoln pulling one over by claiming to be conservative while making progressive moves, it doesn't change the fact that the republican party was a conservative party, led by a conservative identifying person.