There are a great many people on the left of American politics who think that there is one - and only one - lesson of Vietnam: parties fighting unpopular wars lose power, perhaps for a generation. But this is a remarkably solipsistic way of viewing Vietnam. Of the three Presidential elections fought during that war two saw the President re-elected by massive proportions. The 1964 election gave the Democrats their biggest victory since FDR and 1972 their biggest defeat since Lincoln. (And the latter may not count for much, as the most staunchly Democrat states sat that one out).
Certainly, if you discount 1964, when the war had hardly begun, the Democrats did badly during Vietnam, but that is not the same as saying it was politically disastrous for governing parties.
In fact there is another lesson from Vietnam: divided parties lose. The Democrats were broadly united in favour of the war in 1964, and won, big. By 1968 they were divided on Vietnam and on civil rights. They lost, garnering both second and third places in a system that rewards only first. In 1972, still divided, they lost again. And the common factor was Democrat divisions. In 1968 the party halfheartedly supported a pro-war ticket and in 1972 the party halfheartedly supported an anti-war ticket. What they should have learned is that neither half of their party could beat the Republicans alone.
But they seem determined to learn a different lesson. Instead of celebrating a diverse series of views about what they would do instead, and focussing on attacking missteps by the President, they are engaged in internal bickering about their own purity. De-selecting the man who was, just six years ago, their choice for Vice-President is going to reinforce the established public view that the Democrats are deeply divided.
In 1970 the Democrats made an impressive 12 seat gain in the House (in 2006 they need 15 seats to gain control). In the Senate they lost three seats (compared with the six they need to gain in 2006). This mixed result hardly foretold the the upcoming disaster. I doubt they will get the message this year either.