In 1968, both Nixon and Humphrey were not liked
Nixon was not particularly admired, having lost the presidential race in 1960 and the race for governor of California in 1962. His opponent in the 1968 campaign, Hubert Humphrey, was also not liked. He was seen as a stooge for Lyndon B. Johnson, who declined to run for the same reason as Truman – because he knew he wouldn’t win. Humphrey was Johnson’s vice president, and supported the war. Therefore, the Democratic Party was torn. If they voted for Humphrey, they would be supporting someone who betrayed them. If they voted for Richard Nixon, they would be supporting a man they despised (and regarded as part of McCarthy’s witch hunt). The Republicans could live with Nixon but no one really liked him. He won, but by a very small margin in the popular vote. Then there was Watergate, but that’s another story. The point is that paranoia, and worse, violence, have been seen in presidential races before. The issue is whether there is any commonality. There is one. The tensions around the 1952 race came after years of frustration in Korea. The 1968 race came after years getting bogged down in Vietnam. 2016 comes after 15 years of war in the Middle East and Afghanistan. It would seem that when there is an extended and large-scale war that appears to have no end, what we might call extreme elections follow. It is the only common denominator I can see.
But it is a reasonable one. The United States sees itself, reasonably, as a powerful country. When it goes to war, there is an expectation that it will be successful. When it is not successful, people search for an explanation or demand that the war be ended.
There are frequently two camps. First, there is the camp that wants to be more aggressive. Second, there is the camp that wants to withdraw. There is also the camp that wants to find the culprit who drew the U.S. into the war, and the culprit who caused us to lose.
In Korea, Douglas MacArthur, who led the United Nations forces in the war, wanted a more aggressive policy. Joseph McCarthy wanted to identify the people who caused the U.S. to lose, while many on the left wanted the U.S. to leave.
In Vietnam, Barry Goldwater, Republican candidate for president in 1964, wanted the U.S. to be more aggressive. Many Vietnam veterans wanted to know who caused the U.S. to lose, and the anti-war movement wanted the U.S. to leave.