In the long list of politicians, corporations and publicity stunts that have distorted Martin Luther King Jr’s message, the truck advertisement was among the more egregious.

The civil rights leader’s voice boomed out at Super Bowl viewers during a Dodge Ram commercial that used audio from a speech addressing people who aspire to greatness.

Not included in the ad were the sections of the speech in which King warned against the excesses of consumerism and cautioned that “we are so often taken by advertisers”. That criticism was evident in an earlier letter to his future wife, saying capitalism had “outlived its usefulness” and existed to take “necessities from the masses to give luxuries to the classes”.

For historians and activists who have studied King’s legacy or spent formative years in the civil rights movement, similarly skewed invocations of King are a familiar occurrence.

Fifty years after King’s death, they say, the image people conjure – of a soothing centrist committed to moderation – belies the radical views King espoused and the criticism he faced before his conversion into an uncontroversial symbol.

“Dr King would no doubt be surprised to find how popular he’s become since he was buried,” said Gerald Horne, a professor of history and African American studies at the University of Houston. “If you look at the polls in April 1968, at the time Dr King was assassinated, you will find he was not as popular then as, apparently, he is now.”

US race relations: The enduring legacy of Martin Luther King Show all 2 1 /2 US race relations: The enduring legacy of Martin Luther King US race relations: The enduring legacy of Martin Luther King April 1965: Dr Martin Luther King (1929-1968) addresses civil rights marchers in Selma, Alabama Keystone/Getty US race relations: The enduring legacy of Martin Luther King Martin Luther King III has taken on his father's work and isactive in helping those in poverty and those who are on the margins of society. His father, who was killed in 1968, was a hero to many, but just plain Daddy when he was at home DAVID SANDISON

That posthumous popularity has made King a go-to example for those questioning the tactics of a new generation of activists.

After chiding American footballer Colin Kaepernick for creating “more divisiveness, more division” by kneeling in protests during games, Clemson Tigers coach Dabo Swinney cited King as a counterexample. Former Arkansas governor Mike Huckabee said King would be “appalled” by the Black Lives Matter movement decrying police brutality.

Last year, amid a national uproar when Donald Trump equated white supremacists to counter-protesters, the conservative Wall Street Journal editorial board criticised “identity politics” that fell short of “Martin Luther King Jr’s language of equal opportunity and colour-blind justice”.

“Now he’s being invoked by conservatives for various reasons, even though their ideological ancestors were opposed to his project,” Mr Horne said. “Dr King became a cudgel by which his descendants can be assaulted and assailed”.

March For Our Lives: Martin Luther King's grandaughter Yolanda's speech in Washington DC

While King is widely lauded now for his calls to racial harmony, his embrace of nonviolence and his pivotal role in the passage of federal civil rights legislation, he was a far less unifying figure in the years before his death.

His ardent opposition to the Vietnam War – amplified in a speech in which he said “these are revolutionary times” and heralded the toppling of “old systems of exploitation and oppression” – led Life magazine to lambaste him for “a demagogic slander that sounded like a script for Radio Hanoi” and suggest he risked ”betraying the cause for which he has worked so long”.

“Achieving civil rights reform made the nation somewhat complacent about human rights issues. It’s almost like ‘well, we’ve had enough change for a while and now we need to go back to normal life’,” said Clayborne Carson, the head of the Martin Luther King Jr Research and Education Institute at Stanford University.

King’s political sympathies drew the attention of the Federal Bureau of Investigation, which looked into his potential communist ties. The bureau ultimately sent King an unsigned letter calling him evil and encouraging him to kill himself.

He was also sceptical of the centrist establishment. In a letter from Birmingham Jail, he assailed white moderates who, more so than virulent racists, represented “the Negro’s great stumbling block in his stride towards freedom” in their hesitation and distaste for disruptive tactics. He linked criticism of his methods to those who called Jesus Christ’s beliefs too extreme.

“Though I was initially disappointed at being categorised as an extremist, as I continued to think about the matter I gradually gained a measure of satisfaction from the label,” King wrote.

After King expanded his focus from the south to the rest of the nation, speaking about issues like housing and employment discrimination, Carson says, newspapers began using a new term: “white backlash”.

“Most white people in the nation did not really have a stake in the southern Jim Crow system, but they had a stake in keeping their neighbourhoods white and in keeping their kids in mostly white schools,” Mr Carson said.

Over the decades, King’s martyrdom helped transform him into a universally lauded figure. Yet even that process required overcoming resistance, Horne says, noting the long struggle to enshrine an official federal holiday honouring King.

“Finally the dam broke and the King holiday received general acceptance,” Mr Horne said, “but that meant the price of the ticket was diluting the King message so that it could be acceptable in a country where conservatism was a major force.”

Given King’s trajectory, Mr Carson said, his conversion into a beloved and unifying figure was not inevitable.

“I think that if he had lived there wouldn’t be a holiday,” he added. “He would have been too controversial.”

In recent weeks, protests have rocked Sacramento, California, after police officers shot and killed an unarmed African-American man named Stephon Clark. Some residents complained after protesters aligned with Black Lives Matter disrupted traffic and blocked entrances to a basketball game, prompting a rebuke from Reverend Al Sharpton when he spoke at Clark’s funeral.

“Don’t memorialise [King] next Wednesday if you can condemn these kids today,” Mr Sharpton said.

The pushback sounded familiar to Mr Carson.