Why was fdr's supreme court plan criticized

On February 5, 1937, United States President Franklin Roosevelt planned to increase the size of the Supreme Court to 15 judges. Roosevelt’s reason for doing was to make sure that the Supreme Court would be more efficient in carrying out its duties.  It would alter the balance of the court.  Many of the Republicans as well as Democrats in congress opposed this plan.  When the Bill was brought to congress to be voted on, the result was that it was rejected. Roosevelt was also criticized  because of he tried to challenge the Supreme Court’s independence.