Donald Trump defiantly returned Saturday to the site of a campaign rally where an assassin's bullet almost killed him in July, questioning whether his opponents were responsible 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 candidate told tens of thousands of supporters after taking the stage behind bulletproof glass.

Calling the gunman a "vicious monster," Trump vowed he would "never quit... never bend... never break" to cheers of "fight, fight, fight" from the crowd.

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."

"I wasn't supposed to make it," he said ominously.

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

"There's a lot going on that's unnerving," said Heather Hughes, 43, who had traveled 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 on stage, stressing the tight margins that will likely decide the election in battleground states like Pennsylvania and encouraging voter registration.

Trump "must win to preserve democracy in America," said Musk, echoing the alarmist messaging he frequently pushes to his 200 million followers on his X platform.

- Race upended -

Much has changed since Trump's last visit to Butler, when he was riding high in polls after crushing Biden in a TV debate.

Barely a week after the failed assassination bid, the presidential race was turned on its head when Biden dropped out and was replaced as the Democratic nominee by Vice President Kamala Harris.

Harris has clawed back the poll deficit -- reversing it in some states -- and the seismic events in Butler that had threatened to unleash chaos were largely overtaken.

Trump's rally appeared aimed at recovering the momentum as a bruising campaign enters its final phase.

Harris, who Trump called incompetent, was in North Carolina Saturday meeting first responders and people hit by Hurricane Helene, which killed at least 220 as it ripped through the southeast.

The emergency response is "an example of the best we can do when we bring resources together at the federal, state and local level and tap into the kind of collegiality that produces results," she told officials at a briefing.

Trump has criticized the federal response, alleging without evidence that the Biden-Harris administration wrongly redirected relief funds to migrants.

- Election violence? -

The heavy security presence on Saturday highlighted persistent security fears, underscored when another attempt on Trump's life was foiled last month.

The Secret Service has been pilloried for failing to secure the building where the Butler shooter managed to fire at Trump eight times before being shot dead.

Along with Trump, two supporters were wounded and one -- firefighter Corey Comperatore -- was killed.

Trump and his campaign have sought to challenge Democrats' warnings about the former president representing a threat to democracy.

"The Republicans aren't violent... I think they (the Democrats) incite. They keep talking about Hitler and the end of democracy," said retiree Glen Scheirer, who was with five relatives wearing identical "By the grace of God" T-shirts showing Trump after the shooting.  

In the immediate aftermath of the Butler incident, all sides urged a lowering of the political temperature. 

But Trump quickly reverted to his signature inflammatory rhetoric and has refused to commit to accepting the eventual result in November.

He has been indicted over the effort to subvert the 2020 election that culminated in his supporters storming the Capitol.

Asked about the possibility of further election-related violence, Biden said Friday he did not know if the vote would be peaceful.