David Greenberg has an ancient T-shirt that he likes to wear in summer. It bears the slogan: “Kennedy for President.” But this year he had to take it off.

“I couldn’t wear it. I kept getting odd looks. The shirt took on new meaning,” says the Rutgers University professor of US history.

The T-shirt was a memorial to the great hero of the Democratic Party, President John F. Kennedy, who was assassinated in 1963. Although it might equally refer to his brother, Robert F. Kennedy, another darling of the Democrats, who also fell to an assassin’s bullet in 1968 when he was running for the White House.

But the shirt’s meaning was overtaken in recent months by the political campaign of RFK’s son, the last of the Kennedy dynasty in national politics, Robert F. Kennedy Jr, 70, who decided to run for the presidency this year as an independent.

At its peak, his candidacy was polling around 15 per cent nationally, enough to be a serious spoiler for the main party candidates.

RFK Jr, as he’s known universally, is blessed with the family’s good looks but none of its, ah, good sense. He blamed a “brain worm”. No, not a worm as metaphor for a mental quirk but a brain abnormality that he said was caused by “a worm that got into my brain and ate a portion of it and then died”.

Many Americans might have overlooked this, except that he fell into a flirtation with conspiracy theories, a flirtation that became a fixation. Most damagingly for his public standing, he promoted misinformation about COVID vaccines.

But last week, RFK Jr announced that he was suspending his campaign as an independent. He would support Donald Trump instead.

Dr Donald “bleach” Trump embraced the anti-vaxxer on stage at a rally on Friday and welcomed his support. He’d earlier called him “the dumbest member” of the Kennedy family, but last week lauded him as “a brilliant guy”. Trump suggested that he might give him a job in a Trump administration.

Five of RFK’s siblings mourned this new alliance in a statement as “a sad ending to a sad story”. Why sad? Because it was “a betrayal of the values that our father and our family hold most dear”.

As Professor Greenberg says, the Kennedy name is synonymous with John F. Kennedy’s “restoration of an optimistic, liberal, can-do politics to the White House in 1961, and Bobby’s campaign represented something similar” in 1968, the spirit of so-called Camelot.

“I think all these ideals have clung to the Kennedy family even though it’s suffered numerous scandals,” says Greenberg. “It was a myth, but it was a myth that was not without some substance.”

“What was disturbing is now depressing,” as RFK Jr’s kookiness gives way to political, ideological and policy faithlessness in the arms of Donald Trump.

One of the Kennedy dynasty’s signature issues has been environmental protection, a priority shared by RFK Jr for decades.

A fellow environmental activist, Michael Brune, stood shoulder-to-shoulder with RFK Jr when the two opposed the construction of a super-controversial oil pipeline, Keystone XL.

Both were arrested at a protest outside the White House in 2013. When Trump was made president, he gave the go-ahead to the pipeline; as soon as Joe Biden was made president, he cancelled it.

By now supporting Trump, RFK Jr is joining a candidate whose biggest applause point at rallies is “drill, baby, drill!” as he promises a free hand to fracking and oil exploration.

“It’s sad,” says Brune, who went on to become head of the Sierra Club environmental group, “and surreal”.

Surreal indeed. RFK Jr keeps an emu among other exotic pets, and he recently confessed to leaving a dead bear cub in Central Park years ago as a prank. When the major parties excluded him from the recent Biden versus Trump debate, he debated himself. While the president and former president debated on a TV screen, RFK Jr watched and pressed “pause” every now and then to insert his own views for his assembled fans.

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Why did he drop out? Partly because he was a victim of Kamala Harris’ campaign. She drew about half his support base to the Democrat ticket. And partly because he was having trouble getting his name onto ballot papers in many states; he managed to make it in only 26 of the 50 states.

So how will his remaining followers – perhaps 5 per cent or so of the likely electorate – cast their votes in the vital “swing” states, where his name will not appear on the ballot? Will they vote for Trump or Harris? A poll on the weekend by New York Times-Siena College showed that they were split right down the middle, with 35 per cent going to Trump and 34 per cent to Harris.

Most are likely to stay home. Pew Research Center points out that his supporters were only ever feebly attached to him in any case – fewer than a quarter were “extremely” motivated to vote even when he was still running, whereas the corresponding figure was 70 for Harris and 72 for Trump.

The expert consensus, as distilled by Larry Sabito of the University of Virginia, is that he will bring to the Trump voting tally “at most a fraction of a per cent”. RFK Jr seems destined to be a memorable figure in US politics, but an inconsequential one.

After the two Kennedy brothers were murdered, the third Kennedy brother, Ted, was the family champion in American politics. “The dream shall never die,” he declared famously.

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But now? “RFK Jr can’t single-handedly determine the famous Kennedy legacy,” says Greenberg. It’s a big clan.

Today there are two of the family in government service – Caroline, as ambassador to Australia, and Joe Kennedy III, as special US envoy to Northern Ireland. But there is a rising generation and some promising talent, including Caroline’s son, journalist Jack Kennedy Schlossberg.“But,” concludes Greenberg, “for now the dream has taken a hit.”

Peter Hartcher is political editor and international editor of The Sydney Morning Herald and The Age.