I've heard and read that if you look at things constitutionally, the 2 worst presidents by far would be Lincoln and FDR.

However, as far as destroying (changing) the union they were fighting for, I don't think the Union soldiers who were fighting to preserve the Union would agree with you. They were fighting the keep all states in the Union (at least this is what my ancestor wrote in his letters.) Apparently they were not considering the "type" or "form" of government, but rather the country as a whole.