When? Well, as posters have pointed out, there was Animosity towards Obama, and other have pointed out that Bush wasn’t all that liked either, nor Clinton, nor Bush sr, and not reagan even. Carter was a divisive figure, and Nixon destroyed democracy. LBJ stirred up quite a bit of resentment with the Civil Rights Act, and someone hated JFK enough to kill him. Everybody loved Eisenhower, of course, and Truman was there for the end of the war. No one agrees on whether Roosevelt saved the economy from the depression or if his policies exacerbated it, though many blame Hoover because it started on his watch. No one who knows who Coolidge is likes him, and Harding didn’t make it very long.
Well, that makes up most of last century, we must have gotten along better before that. Of course, there was a bit of a war when we were unable to settle our disagreements and instead found enough animosity for other americans to go out with the intent to kill them.
The civil war wasn’t the first schism between Americans, we almost went to was 30 years earlier over disagreements of the nature of handling other human beings.
Ah well, lets go all the way back to the founding fathers, they all got along great, didn’t they? John Adams, Thomas Jefferson, Aaron Burr, and Alexander Hamilton were all best of friends, right?
I think the question in the OP is backwards. We were founded by different cultures from different countries with different reasons for coming. We have never been a cohesive monolithic culture that has agreed on pretty much anything.
The real question that should be asked, is when and how were we able to put aside our differences, forget our hate, and work together to do the great things that America has achieved.