Half a century ago, Lyndon Johnson had gone from “Uncle Corn Pone” — the derisive nickname given him by the Kennedys — to a record of domestic achievements rivaled in the 20th century only by Franklin Roosevelt.

And on March 12, 1968, it all came crashing down, thanks to a Minnesotan.

Sen. Eugene McCarthy scored 42 percent of the vote in the New Hampshire primary that day, a shocking result that pierced the illusion of Johnson’s omnipotence. The Texas political giant left the presidential race by the end of the month.

“No one thought he would get 10 percent,” said Vance Opperman, a Minneapolis attorney who was the Hennepin County DFL chairman at the time and a McCarthy activist.

McCarthy, a World War II codebreaker and St. John’s graduate, was an unlikely slayer of giants. He was an intellectual, a poet, a wit.

Norman Mailer said he didn’t seem like much of a political leader; “not that tall, tired man with his bright subtle eyes which could sharpen the razor’s edge of a nuance, no, he seemed more like the dean of the finest English department in the land.”

In a famous speech nominating Adlai Stevenson for president at the 1960 Democratic National Convention, McCarthy roused the delegates with oratory that to modern ears sounds foreign, infused as it was with his classical education and elegiac voice: “And I say to you that the time has come to raise again the cry of the ancient prophet. And what did he say? He said the prophets prophesy falsely.”

McCarthy, Opperman said, was “not the guy who fixed someone’s Social Security appeal.”

Regardless, what really mattered: McCarthy showed courage when most in Washington were silent, despite the evident strategic drift — and the body bags returning from Vietnam.

“He knew the war was false and amoral, and he stood up and he said it,” Opperman said. “And he was the only one.”

For Opperman’s generation, the war was no abstraction. “A lot of people were driven to this campaign because they didn’t want to go to Vietnam and get killed. They knew people who had gone and not come back,” he said.

As a law student with a young child, Opperman was safe from the draft, so he gave up his slot at the Democratic National Convention in Chicago to a friend — so the friend could get a draft deferment.

With Johnson out of the race, another Minnesotan was suddenly front-and-center: Vice President Hubert Humphrey, the former mayor of Minneapolis and great civil rights champion who suffered at Johnson’s side, was now in the race.

“There was a feeling that this changes everything,” said John Stewart, Humphrey’s legislative director.

First, tragedy would befall the nation twice more that spring with the assassinations of the Rev. Martin Luther King Jr. and Robert Kennedy.

The race came down to McCarthy and Humphrey. But despite McCarthy’s successes in the Democratic primaries, that’s not how you won the nomination in those days.

The power was all in the literal smoke-filled rooms, where the big city machine bosses and union presidents gathered to hash out the nomination. They favored Humphrey.

To Humphrey supporters like Norman Sherman, his press secretary, there was no question who was the better leader.

“It is inevitable that McCarthy and Humphrey appear to be equals looking back — two giants in the Senate. They were not,” Sherman said. “Humphrey produced legislation across a broad spectrum. McCarthy did not.”

To this day, those still alive in the Humphrey camp are bitter that Johnson wouldn’t allow Humphrey to truly break with his Vietnam policy until it was too late in the campaign.

Nor does McCarthy escape their ire: He held his endorsement until the end, and it seemed less than full-throated at the time.

“We went from 18 points behind after the convention to a loss of less than 1 percent. Numbers were there in the right places,” Sherman said. “McCarthy had told Humphrey he would endorse a couple of weeks after the convention, giving his people time to heal. He waited and provided only a chicken-[expletive] endorsement.”

Richard Nixon became president, and 21,000 more Americans died in Vietnam before it was over.

“The stakes were not insignificant, and McCarthy should have realized that,” Stewart said.

According to John Tirman, executive director of MIT’s Center for International Studies, estimates of Vietnamese military and civilian deaths in the war range from 1.5 million to 3.8 million.