This was already discussed in another thread. Truman holds the record for the lowest approval rating in modern opinion polls. He had 23% after he fired Gen. MacArthur during the Korean War. Nixon is a close 2nd with 24% just before he resigned.
Boris Yeltsin had an approval rating of 8% a short time before he won re-election against the communists in 1996. Some sources on the net cite his rating at the lowest was a mere 2%