Word Detective offers this thorough definition:
‘To throw someone under the bus’ is defined as meaning to sacrifice; to treat as a scapegoat; to betray, but I think the key to the phrase really lies in the element of utter betrayal, the sudden, brutal sacrifice of a stalwart and loyal teammate for a temporary and often minor advantage.
Not all our popular phrases come from the King James Bible or Shakespeare – there is, apparently, no antecedent phrase about throwing someone under the wagon. This phrase belongs to us, so you’d think its origin would be clear. You’d be wrong.
Merriam Webster says:
The origins of throw someone under the bus have been attributed to minor league baseball, Cyndi Lauper, the slang of used car salesmen, and various other improbable sources…
The 1984 quote from rock star Cyndi Lauper where she uses the phrase “under the bus” (without “throw”) may or may not count as a sighting, according to Word Detective.
But Merriam Webster attributes the earliest written usage to Elinor Goodman, Financial Times (London, Eng.), 10 Dec. 1980:
Some still pin their hopes on the “under the bus” theory which has Mr. Foot being forced by ill health—or just the pressures of the job—to give way to Mr. Healey before the next election.
This, however, lacks the malevolent flavor of current usage. A better citation comes from Julian Critchley, The Times (London, Eng.), 21 June 1982, but a completely satisfying citation has eluded me.
So I must conclude that, while a recent phrase generates a lot of possibilities, it isn’t any easier to pin down than a venerably aged phrase.