Obama is a relatively young man for a prospective president. He would have been too young during the 60's to be caught up in the pro and anti-Vietnam war sentiments that divided the country at the time, and that still affects those in the nation who lived through it - he is able to step aside from, and beyond, the divisions wrought by that debate. For example, Iraq opens the wounds again, since it seems to go on and on, just like Vietnam, and again as with Vietnam, should or should not America be involved in their politics? In politics, the Democrats are the liberals, and the Republicans are the conservatives. And it was these two sides of the fence that were so opposed in the 60's. The 'poisonous legacy' I think is this sense that there is still a strong split in the nation between liberals and conservatives stemming from the 60's.
An American who lived through it will hopefully give you a more 'eye-witness' understanding of this. I'm sure someone else will also post a response.
Student or Learner