No need to go in to the whole "classical liberalism (aka libertarianism) is the real liberalism" show. The Republican Party circa 1860 was, for its time, a liberal party in the modern sense of the word.
Bullshit.
If they were "liberal" they would have identified themselves as such, or others would have identified them as such.
People think that their own warped and incorrect understanding of things actually dictates history. And people are wrong. History is what it is. Republicans were NOT liberal, they never were. This is just more adjustment of history by a group of people who REALLY don't like to be identified as what they are (fascist) and desperately try to force people to associate them with a different set of values than those they have and promote.
LIBERALS have nothing against slavery, they never have. LIBERALS believe that certain groups of people have "more" rights than others, and think that a person's intentions should be able to justify any sort of behavior they choose to inflict upon others. You see it in their insistence that people aren't responsible for their own behavior...they will always find a good "reason" for bad behavior, that eliminates the need for punishment and will establish an excuse for tyranny.
Today's liberalism was birthed in the 1930s, and is NOTHING but an offshoot of communism.