Forty years ago, the Democrats met in Chicago, their most disastrous convention ever. Denver obviously won't be a repeat, but Democrats face some similar dangers if they don't pull the party together. I know this from personal, if youthful, experience.

In 1968, I was a 10-year-old Chicagoan, fascinated by politics, determined to hang out at the convention. My mother was working for Vice President Hubert Humphrey and my father for Sen. Eugene McCarthy, both of whom had their convention headquarters at the Conrad Hilton Hotel on Michigan Avenue, not far down the street from where Barack Obama's headquarters is today. It was late August, only a couple of months after the assassination of Bobby Kennedy, and even before Chicago police started clubbing reporters and demonstrators outside the hotel, the mood was tense.

Humphrey, the presumptive nominee and soon-to-be Senate tutor of a 30-year-old freshman named Joe Biden, had entered no primaries, but he had the support of President Johnson, as well as the urban bosses who still ran the party. His slogan, "the politics of joy," was especially off-key that year, as a bitter struggle over the Vietnam War swept the country. Humphrey had started out as a Minnesota pharmacist, so someone got the dumb idea of having part of his headquarters decked out like an old-fashioned pharmacy. My mother, who knew Humphrey, volunteered there, running the pharmacy that sold Humphrey buttons, posters, scarves and other memorabilia. Business wasn't brisk....

As Rick Pearlstein recounts in his new book, "Nixonland," Humphrey left the convention trailing Richard Nixon badly in the polls. It was the worst "reverse-bounce" in history. But after he broke with LBJ on the war in a speech in Salt Lake City, Humphrey began quickly closing the gap. He lost in November by a hair.

One of the reasons for Humphrey's loss was that many McCarthy supporters (not my father) refused to close party ranks and support him. They were so angry over the war and their personal bitterness over the way McCarthy was treated that they sniped at Humphrey publicly and stayed home in November.

Some of these people—then in their mid-20s or 30s, and now in their mid-60s or 70s—are the same liberal Democrats who say they cannot vote for Obama. Their long hair is gray or gone and they certainly aren't sleeping in the park, but the anger they feel resonates of 1968. They might want to consider what happened to the United States after Nixon was elected. It wasn't pretty.