Yes the Republican party was anti-slavery in the 19th Century.  In the 20th Century they abandoned their principles in order to gain power. They sold their souls and became the party of segregationists and white supremacists. 

The Democrats moved in the opposite direction, becoming the party of civil rights. That's how we got to where we are today. We can't change our past, but we can change our future.  One party is on the right side of history, and it ain't the GOP.