On April 4, 1968 Dr Martin Luther King Jr was assassinated while leading a bold effort to teach America an urgent lesson: Racism is not just the boot on the neck of people of colour, it is also the great wedge that divides Americans. And everyone who gets divided loses.

On December 4, 1967 King announced a multiracial ‘Poor People’s Campaign’ that would march on Washington, DC, that summer.

The idea gained traction as groups of poor whites, Asian Americans, Latinos, and indigenous people joined the campaign being organised by King and Southern Christian Leadership Conference.

In promoting it King would decry the “idle industries of Appalachia” in the same breath as the “empty stomachs of Mississippi”. The reality, King made clear, is the economic value of poor whites’ labour had been depressed since the days of slavery by the forced labour and continuing oppression of black people. The divided get conquered.

That idea that working people of all races had common interests to fight for threatened — as it still does today — the old colonial system of divide and conquer that allowed King George and every would-be American oligarch since to extract massive wealth by enforcing massive poverty.

Four months to the day after he announced his Poor People’s Campaign — 55 years ago — King was assassinated on a balcony at the Lorraine Motel in Memphis, where he had travelled to stand with striking sanitation workers fighting for decent working conditions.

It’s telling that after all he had been through fighting Jim Crow and segregation — the bus boycott, the first March on Washington, passage of the Civil Rights Act — King was murdered fighting to unite working people across racial lines.

He wasn’t alone. Robert F Kennedy was gunned down not long after as he ran for president on a similar platform.

Even before King and Kennedy, Harry Moore and his wife were blown up in their home on Christmas 1951 by the Ku Klux Klan. The Florida National Association for the Advancement of Colored People leader was organising the Progressive Voters League, seeking to unite Floridians across racial lines and had just led an effort that registered one million new voters. Even Malcolm X was assassinated after he returned from Mecca and said unity across racial lines was possible.

Killing those who would unite us is an American tradition older than our nation itself. The first revolt by American colonists was in Gloucester, Virginia, more than 100 years before the Declaration of Independence. Indentured Europeans and enslaved Africans organised to rise up against cruel Virginia plantation owners. The organisers were hanged.

Two years to the day after King announced the Poor People’s Campaign, Black Panther Fred Hampton was leading a “Rainbow Coalition” of blacks, Hispanics, Asian Americans, and poor Whites in Chicago when he was murdered — premeditated and carried out with military precision — by local police.

It’s as true today, as it was in 1968, that there are almost twice as many whites trapped in poverty as blacks. The fact that the nation’s news media render the white poor invisible doesn’t change the facts.

That so many of us still tolerate millions of Americans of every colour being trapped in poverty is a factor in the toxic tensions that threaten our domestic tranquility.

It is also proof we never actually learnt the lesson Dr King gave his life trying to teach us.

If you ever forget the logic of King’s final strategy, just pull out a US$1 bill and turn it over. It’s right there in the great seal of the United States, albeit in Latin: “E pluribus unum”, “Out of many, one”.

Ben Jealous is executive director of the Sierra Club, the nation’s largest and most influential grassroots environmental organisation. He is a professor of practice at the University of Pennsylvania and author of Never Forget Our People Were Always Free.