How did the two major political parties in America, the Republicans and the Democrats, become the way they are today?

The Republican Party, for instance was founded by anti-slavery activists and modernizers, which in a way is opposite of the conservative party we see today.

The Democratic Party, on the other hand, for a long time fought for a weak federal government and strong adherence to the constitution. Today, they are the liberal party amongst the two.

Could anyone give an overview over why and how this happened?