Donald Trump made a defiant return on Saturday to the venue of a campaign rally where an assassin's bullet narrowly missed killing him in July, questioning whether his political opponents tried to take him out and declaring he would "never quit".

"Exactly 12 weeks ago this evening, on this very ground, a cold-blooded assassin aimed to silence me," the Republican told thousands of cheering supporters after taking the stage behind bulletproof glass.

Calling the gunman a "vicious monster," Trump vowed he would "never quit... never bend ... never break".

Trump's much-hyped return to Butler, Pennsylvania, came exactly one month before the November 5 presidential election, the outcome of which President Joe Biden suggested on Friday might not be peaceful.

Trump lashed out at his political opponents, calling them the "enemy within" who had pushed to indict him and "who knows, maybe even tried to kill me".

Security was noticeably tighter than Trump's July rally, with sniper squads positioned on several surrounding buildings, and a surveillance drone deployed overhead.

"There's a lot going on that's unnerving," said Heather Hughes, 43, who had travelled from New Castle in must-win Pennsylvania.

"Do I think he's safe? No, I think there's going to be another attempt. But I think he's going to make it through."

After the assassination attempt, pictures of Trump – with a blood-streaked face, pumping his fist and shouting "fight, fight, fight" – became defining images of the campaign.

On Saturday, many Trump supporters wore shirts emblazoned with the iconography, and some sported ear coverings recalling the bandage the former president wore after the shooting.

Billionaire Elon Musk joined Trump onstage, stressing the tight margins that will likely decide the election in battleground states like Pennsylvania and exhorting the crowd to register to vote.

Trump "must win to preserve democracy in America," said Musk, echoing the alarmist messaging he frequently pushes to his 200 million followers on X, the social media platform formerly called Twitter, which he owns. (AFP)