The support for the New Deal art programs declined because these arts celebrated the progress made under Franklin Roosevelt and promoted the president and his programs.Most New Deal artists were grateful to President Roosevelt for giving them work and fully supported the New Deal's liberal agenda.
But when Harry S Truman was elected the new president,he obviously did not like the New Deal artwork because they were being used to praise Roosevelt.So President Truman stopped all funding for New Deal artwork and thus support for the New Deal art programs declined.