From JFK to Trump, assassinations and attempts are rife in US ...

Prior to the attempted assassination of former US president Donald Trump, multiple US presidents and presidential candidates have been the target of political violence.

Four past US presidents were killed while in office, while attempts were made on the lives of two others.

Political scientist Richard Herr said the level of violence feels "reminiscent" of the deep divisions in the US during the 1960s. 

The attempted assassination of former president Donald Trump at a packed rally in Pennsylvania has shocked the world, but United States' presidents are no strangers to attempts on their life.

While the last attempt on an American president was more than 40 years ago, the country's political history is littered with assassination attempts, some of them successful, all of them involving guns.

Death threats unfortunately appear to be par for the course when it comes to the presidency of the United States but the last president with a known attempt on their life was Republican Ronald Reagan.

It was March 1981, mere months after he was first elected to the White House; President Reagan was leaving a speaking engagement in Washington when a lone gunman fired on him. A bullet hit him under the left arm.

Police and Secret Service agents reacting during the assassination attempt on US president Ronald Reagan outside the Hilton Hotel in Washington, DC, on March 30, 1981.(AFP)

But the motivation was not political, explained ANUI International Relations Professor Wesley Widmaier.

"The Reagan shooter, he was a mentally disturbed individual who had seen the movie Taxi Driver with Robert De Niro and Jodie Foster," he said.

"He was obsessed with Jodie Foster and there was a shooting of a political candidate in the movie. So he thought he'd emulate the move.

"It had nothing to do with the the kind of partisan agenda of Reagan or anyone like that."

Reagan was taken to hospital and less than two weeks later was able to return to the White House, but the incident "really weakened him".

"It had a much greater effect than people realised as the time on Reagan as a person," Professor Widmaier said.

"To the point where he was never really the same afterwards in terms of his rigour and his energy."

Reagan would serve as president until 1989.

Political scientist Richard Herr remembers waking up at 2am in 1968 and wondering why they were showing footage of President John F Kennedy's assassination.

"I was asleep in Nebraska, two in the morning, I woke up feeling something was awful and turned on the news, and I thought, 'Why are they replaying Kennedy's assassination?'"

The 35th president, who was the youngest man elected to the office, had been killed five years earlier on November 22, 1963, in Dallas, Texas.

But this footage was not of John F Kennedy.

"It was his brother Bobby, that was shot in California," Dr Herr remembered.

Robert F Kennedy, a US senator, was campaigning for the Democratic Party's presidential nomination in June 1968 when he was assassinated.

"If Bobby Kennedy hadn't been shot, and he ran and won the Democratic nomination, he could have been president in 1968 and history would have gone in an entirely different direction," Professor Widmaier said.

Four years later, in 1972, an attempt would be made on the life of another Democratic presidential hopeful, George C Wallace.

Wallace, a staunch believer in segregation, survived but would spend the rest of his life in a wheelchair. He would never be the same again.

"Interestingly enough, if we look at Wallace and Trump because both of them have [promoted] violence — the use of axe handles to stop segregationists in the 60s and Trump's comments, 'Don't be too careful when you arrest protesters,' and 'You know what I'd like to do with him,' sort of comments. Wallace actually changed after the assassination attempt in 72 that left him in a wheelchair.

"He said he was a born again Christian, and he regretted his segregationist policies.

"Whether [the Pennsylvania shooting] will encourage Trump to dial down his level of violent [rhetoric] we'll have to wait and see. "

Dr Herr said the level of violence and division in the US at the moment felt like being back in the 60s all over again.

"The social division, the profound division within America now, over the last eight years is very reminiscent of the kind of deep divisions of the US in the 1960s," Dr Herr said.

Professor Widmaier agreed it felt like America was heading back to the 60s.

"Everyone talks about the 60s as kind of shorthand for this political upheaval and assassinations and political violence, almost entering the mainstream from both sides," he said.

"When you get, which I don't think you've had since the 60s, this intensification of political rhetoric … extreme rhetoric leads to extreme beliefs, leads to extreme actions. That's why you see something like today."

Before the 60s there were numerous attempts on the lives of American presidents, but they were spread out over decades.

Puerto Rican nationalists tried to assassinate president Harry Truman in 1950, Franklin Roosevelt was president-elect in 1933 when he was shot at. While in 1912, Teddy Roosevelt was shot and continued speaking.

Others have not been so lucky. President William McKinsley, who was president from 1897 to 1901, was killed in office; while Republican President James Garfield spent just 200 days in office before being assassinated in 1880.

The first and most famous assassination was Republican President Abraham Lincoln in 1865.

"Lincoln is obviously the big one," Professor Widmaier said.

"It changes the course of American history."

The assassination of America's civil war president Abraham Lincoln might be history's most notorious.(Wikipedia Commons: Currier & Ives)

Elected into office in 1860, President Lincoln made it his mission to end slavery in America but civil war broke out the following year as the southern states resisted.

In 1863, he issued the Emancipation Proclamation, declaring forever free slaves within the confederacy.

Two years later, he was fatally shot while attending a play.

His successor, southern-based vice president Andrew Johnson, steered the country down a very different path.

"The vice president then tried to roll back the reconstruction agenda of Abe Lincoln and helped to bring about the Jim Crow era in southern politics."