A string of 2016 presidential hopefuls have tried to land a punch on “The Donald” with little effect.

Trump has brushed off earnest-sounding accusations that he is not presidential, largely because many Americans seem to be clamoring for an unconventional leader. Democratic strategists privately admit that casting Trump as a “loose cannon” similarly misfired -- that tack, they say, only played to the Republican’s tough man brand. So in San Diego on Thursday Clinton tried a new formula.

Trump, she argued, is not just unpresidential and unpredictable, he’s too unhinged to be in power, or as she put it -- “temperamentally unfit” to be commander-in-chief.

“This is not someone who should ever have the nuclear codes –- because it’s not hard to imagine Donald Trump leading us into a war just because somebody got under his very thin skin.”

She even suggested psychiatrists explain his “affection for tyrants.”

You could call this Clinton’s “Daisy strategy.”

In 1964, incumbent president Lyndon B. Johnson faced a challenge from a tough-talking Republican nominee. Barry Goldwater was a US Senator and friend to Democrat John F. Kennedy,

but he was also from the right fringe of the political mainstream.

Goldwater was endorsed by the white supremacist Ku Klux Klan, campaigned on a platform of withdrawing from the United Nations, and advocated the use of tactical nuclear weapons against north Vietnam.

His nuclear position prompted Johnson’s campaign to build and unleash “Daisy.”

A minute long, and only broadcast by Johnson’s campaign once, the attack ad would become one of the most famous in US political lore.

It starts with an angelic young girl counting as she picks petals of a flower.

Then, suddenly, an ominous sounding voice from mission control begins a countdown to zero -- which brings a boom, blinding light and a mushroom cloud that would be unmistakable to voters so soon after the Cuban missile crisis. “These are the stakes” says Johnson, playing narrator. “To make a world in which all of God’s children can live, or to go into the dark. We must either love each other. Or we must die.”

- AFP