Why did some critics see Roosevelt's court reform bill as an attempt to constitutional balance of power

Some critics saw Roosevelt's court reform bill as an attempt to upset the constitutional balance of power because it would allow him to add more justices to the bench of the Supreme Court. By adding in more justices (aka judges) to the Supreme Court with similar political views, President Franklin D. Roosevelt would have been able to pass any law he wanted without worrying about it being declared unconstitutional by the Supreme Court. This would negatively effect the system of checks and balances established by our government with the US Constitution.