Why did people regard fdr's court-packing scheme as a threat to the
constitutional principle of separation of powers?

Many people opposed FDR's court packing plan. Many people saw his move as a threat to the principle of separation of powers. People also accused the president of packing the court with people who supported his view. This completely made people not like this even more then they had before, although some people still did approve of FDR's packing plan. Eventually Roosevelt withdrew his plan after fighting for it for a long time, but he gave up.