The Eugene McCarthy campaign (I think accurately) has been credited with accelerating the timeframe of the anti-war movement. At the beginning of 1968, most Americans still supported the course of the Vietnam War, mostly on the principle of not really knowing much about it. "The government says we're winning, so it must be true!" The Tet Offensive did a lot to convince Americans that maybe the war wouldn't be over 'any day now,' and it really stacked with McCarthy's primary challenge to plant the first seeds of 'Middle' American opposition to the war. Now, McCarthy was pretty much the last choice approached by Allard Lowenstein and Curtis Gans to run against Lyndon Johnson (Bobby Kennedy, James Gavin, George McGovern, and Lee Metcalf were all approached first), so let's say McCarthy retires early from the Senate. He was planning to do this IOTL, but delayed it to run as the anti-war candidate. Here he retires even earlier, and goes off to take a teaching position in Minnesota like he wanted.
Now let's assume Allard Lowenstein doesn't just run on his own (ironically, he would've been a better campaigner than McCarthy), and there's simply no anti-war Democrat in 1968. Johnson could stay on as President and feasibly beat Nixon ITTL, thus extending the war under Johnson, but let's say he doesn't run for the same health reasons as IOTL. If Humphrey wins he would probably pull out of Vietnam relatively early, but I can see Kennedy getting suckered into staying in by the military. Of course, Nixon winning anyway ITTL is the most likely means to keep the Vietnam War going besides Johnson running and winning again. Without McCarthy normalizing the anti-war position in 1968, McGovern is likely not the nominee in 1972. In this scenario, if not a majority, then a plurality of Americans could very well continue to support the Vietnam War in the early-to-mid 1970s. While this might not extend the war until the early 1980s, it could do so into the late 1970s.