Then President Richard Nixon discussed a Vietnam exit strategy before the 1972 election, a tape released to mark 30 years since he resigned shows.
President Nixon and Henry Kissinger talked in the Oval Office
US forces had engaged in a huge bombing campaign that year in North Vietnam.
But Nixon told his National Security Adviser Henry Kissinger that even with US backing, South Vietnam was unlikely to survive.
Some historians see the conversation as evidence that Nixon sacrificed US forces in his quest for a second term.
But Mr Kissinger, now a foreign policy consultant, told the Associated Press news agency that it was an informal conversation which did not reflect Nixon's policies.
"Every once in a while he got discouraged and said 'chuck
the whole thing,' but that was never his policy," he said.
Nixon reportedly denied until his death in 1994 that the 1972
election affected his policies in Vietnam.
In the Oval Office conversation recorded by Nixon's voice-activated
taping system, the then president told Mr Kissinger that winning the election was crucial.
"It's terribly important this year, but can we have a viable foreign policy if a year from now or two
years from now, North Vietnam gobbles up South Vietnam? That's the real question," he said.
Nixon started to draw back US ground troops from Vietnam in 1969.
After winning the election in 1972, he agreed in 1973 to bring the rest
of the troops home.
South Vietnam fell two years later to North Vietnamese troops.
In the tape transcribed by the University of
Virginia Miller Center of Public Affairs, Mr Kissinger advised the president that
they could avoid being seen as failures if South
Vietnam held on to its independence for a few years.
"If we settle it, say, this October, by January '74 no-one
will give a damn," Mr Kissinger said.