What was FDR’s “court packing plan” and why was it such a serious mistake?

FDR’s court packing plan was a plan to add new justices onto the supreme court for each justice over the age of 70.5 years. It showed the limit of FDR’s power over the government.
The court packing plan was a reaction on FDR’s part to how the Supreme Court had struck down key parts of the original New Deal by saying that they were controlling too much economics.
FDR was very popular and got reelected and used the court packing plan as a way to threaten the supreme court for not supporting his bills. The court packing plan to add extra justices was a way of FDR establishing that he would get his legislation through somehow. It led to the supreme court not making significant economic decisions in the same way ever since, as it now focuses more on social issues.
Ultimately, the court packing plan showed that despite FDR’s popularity among the people, he was not able to single handedly do what he wanted in the government. Also, the court packing plan was a threat to the checks and balances of the government and turned some Democrats against FDR.