Two reasons why little was accomplished by Franklin Roosevelt's third term:
- The Supreme Court was challenging his decisions and programs.
The Supreme Court consisted of pre-Roosevelt appointees, and many of them didn't agree with his plans. The Supreme Court invalidated several key New Deal measures (A series of programs, public work projects, financial reforms and regulations), and the court-packing scheme, a plan to expand the Supreme Court to as many as 15 judges, to make it more efficient.
2. The political situation in Europe was deteriorating as war loomed.
By the beginning of Roosevelt's third term, the U.S. was still in a neutral position regarding the WWII, while supplied arms and other forms of aids to Britain and its allies. However, this situation entirely changed when the U.S. government restricted supplies exports to Japan, leading to the Pearl Harbor Attack. An attack that made the U.S. declared war on Japan.
As a result, the government spending and funding were taken out of social programs (which had been bringing positive results in Roosevelt's term) and was redirected on war, putting all U.S.'s efforts and resources in the war, therefore the economy started to collapse.