Malzberg, Barry N. "All Assassins"
Divergence: 1960 CE
What if: Nixon was elected president in 1960 and Johnson in 1964 and 1968.
Summary: In 1972, "the senator" runs again. Upset by his change of heart on the Vietnam war, "Lee" decides to shoot him and his running-mate in Dallas.
Published: In What Might Have Been? Volume 1: Alternate Empires (eds. Gregory Benford and Martin H. Greenberg), q.v.; and In the Stone House, q.v.