Billy Graham is partly correct when he states that "I never heard him one time criticize a living President who was in the office at that time." But as with everything Richard Nixon did, there was always something in it for him. 

In 1967, attempting to fashion a middle ground for himself against his presidential opponent Hubert Humphrey, Nixon lobbied Democrat President Lyndon Johnson for tacit support, which he received.

In 1974 after resigning from the presidency, Nixon was heavily indebted to President Gerald Ford for pardoning him, thus allowing Nixon to avoid a criminal trial and possible conviction.

Later, Nixon successfully used other Presidents to rehabilitate his own image, particularly in his occassional advisary role to President Clinton.