Republican presidential candidate Donald Trump has returned to the rural Pennsylvania site where he was nearly assassinated, rallying a large crowd in the critical battleground state one month before the November 5 election.

"As I was saying," Trump said as he appeared on stage, pretending he was resuming the speech that was interrupted when he was grazed in the ear by a bullet on July 13.

In mid-speech Trump called up on stage a powerful ally, Elon Musk, CEO of Tesla and owner of social media platform X. It was his first time at a Trump campaign event since he endorsed the former president after July 13.

Elon Musk speaks during a rally for Donald Trump in Butler. – Reuters

Musk literally jumped up on stage with his arms held high.

"The true test of someone's character is how they behave under fire," Musk said, calling the election a "must-win situation" for Trump and urging the crowd to register to vote.

"Be a pest to everyone you know," Musk said.

Trump saluted firefighter Corey Comperatore, who was shot and killed by the shooter, and two other people who were wounded.

At 6.11pm, the exact time when gunfire erupted on July 13, Trump called for a moment of silence. A bell then tolled four times, once for each of the four victims, including Trump.

Donald Trump pays tribute to Corey Comperatore during his speech at the rally. – Reuters

In his speech, Trump hinted darkly without evidence about facing "an enemy from within" more dangerous than a foreign adversary.

Of the gunman, he said "a cold-blooded assassin aimed to silence me" but "did not stop our movement."

The crowd appeared to be in the tens of thousands, many wearing Trump regalia.

Some chanted the "fight, fight, fight" slogan Trump used to rally his followers moments after he was shot.

Republican presidential nominee former President Donald Trump greets family members of Corey Comperatore as he arrives at Pittsburgh International Airport. – AP

"We're here to say, we can't be intimidated, we can't be stopped," Ohio Republican Senator JD Vance, Trump's vice presidential running mate, told the crowd.

Vance rejected the argument from Vice President Kamala Harris and other Democrats that Trump represents a threat to democracy.

"Donald Trump took a bullet for democracy. What have you done?" Vance said.

People cheered when Trump's plane did a flyby over the rally prior to landing as the loudspeakers played the Top Gun movie theme song.

Footage from the flyby shows the size of the Trump rally. – Reuters

Trailers were lined up around the site as a protective measure to block the surrounding view including the building where the shooter had opened fire.

Adding to the carnival-like atmosphere, three parachutists with American flags streaming behind them hurtled down and landed near the building.

July 13 was the first of two attempts on Trump's life. On September 15, a gunman hid undetected for nearly 12 hours at Trump's golf course in West Palm Beach, Florida, with plans to kill him, prosecutors have said, but was thwarted by a US Secret Service agent patrolling the course ahead of Trump.

Republican officials hope Trump's return to Butler will generate more support among his hard-core followers and drive up turnout for him in Pennsylvania, a state he and his Democratic opponent, Vice President Kamala Harris, see as crucial to winning on November 5.

"I'm going back to Butler because I feel I have an obligation to go back to Butler," Trump said earlier this week.

"We never finished what we were supposed to do."

Introductory speakers at the rally focused on remembering Comperatore.

"We will not let that tragic day overshadow our community's light," said JD Longo, mayor of the nearby town of Slippery Rock.

A memorial for firefighter Corey Comperatore, who died as he shielded family members from gunfire, is seen in the bleachers. – AP

An attendee of the July rally, Shane Chesher, 37, said he would return for what he expects will be an emotional event.

Chesher said he witnessed the assassination attempt from a seat on the stage behind Trump and is still processing what happened.

"Honestly, I don't know that I understand still to this day what we experienced that day," Chesher said.

Returning to the site, he said, "will be spiritual, and I think it will be very emotional, good and bad".

The Butler shooting led to widespread criticism of the US Secret Service and the resignation of its director.

Critics raised concerns about how the 20-year-old suspect, Thomas Matthew Crooks, who was subsequently shot to death by Secret Service agents, was able to access a nearby rooftop with a direct line of sight to where Trump was speaking.

A Secret Service probe found communication gaps and a lack of diligence ahead of the shooting.

Donald Trump grabs his ear after being shot at his rally in Butler, Pennsylvania, in July. – Reuters

In its aftermath, the agency approved extra security measures for Trump, including using bulletproof glass to shield him at outdoor rallies.

Secret Service spokesperson Anthony Guglielmi said before the latest rally that there had been "comprehensive changes and enhancements to our communications capabilities, resourcing and protective operations".

Trump credits turning his head to read a chart on a big video as having saved his life.

With blood dripping down his face, he raised a fist and shouted "fight" to his supporters, a vivid image from that day.

"I love that chart," Trump said when it flashed on screen on Saturday. "Isn't it beautiful."

Hundreds of supporters arrive hours ahead of Trump's return. – AP