But he wasn't even then. He said he was conservative as in conserving the constitution.
“Don't interfere with anything in the Constitution. That must be maintained, for it is the only safeguard of our liberties.” -Lincoln
He rebukes claims made by the Democrats that they are "conservative", arguing instead that the Republicans' position on slavery is in fact the "conservative" policy, as Lincoln claims it coincides with the views of the American founding fathers, who he said opposed slavery. By supporting slavery, Lincoln claims that the Democrats are in opposition to the teachings of the founding fathers and "reject, and scout, and spit upon that old policy, and insist upon substituting something new."
https://en.wikipedia.org/wiki/Cooper_Union_speech