Whoever might replace Roosevelt could end foreign aid through Lend Lease to Europe and probably also the oil embargo on the Japanese, being isolationist. This prevents the attack on Pearl Harbor and for Russia and Britain to suffer through the war in Europe.
Britain starves into an armistice without Lend Lease support by 1942. The Russians take until at least 1946 just to stop the German invasion, but are never able to bring down the Nazis themselves without it. This war will end in a stalemate too with an arms race to follow. Germany controls most of continental Europe without Lend Lease.