How did slavery become a contentious issue in American politics?

>>1365848
If I had to guess, I would say that the biggest problem is that it made the American ideal inherently contradictory. To quote writer Samuel Johnson: "How is it that we here the loudest cries for liberty from slavers?"

>>1365848
like >>1366623 said, it went against many of the ideals trumpeted during the revolution. Otherwise it became an easy way to attack the south during rising tensions between north and south over tariffs and expansion.

Since the 3/5s Compromise, although the precise things being contested varied

>>1366666
the 3/5s compromise wasn't really about slavery though, it was about wanting more representation in congress.