Billionaire businessman Elon Musk joined Republican presidential candidate Donald Trump on Saturday for his election campaign as the former president returned to the venue of a rally where he was nearly assassinated in July.

Addressing his rally, Trump called his would-be assassin a "vicious monster".

He also invited Elon Musk on stage in Pennsylvania's Butler, saying he is an "incredible guy".

"As I was saying," Trump said as he started speaking behind bulletproof glass, pretending he was resuming the speech that was interrupted when he was hit in the ear by a bullet on July 13.

"Exactly 12 weeks ago this evening, on this very ground, a cold-blooded assassin aimed to silence me," he told thousands of his supporters.

Musk backed Trump strongly for the upcoming elections sayings: "We had one president who couldn't climb a flight of stairs and another who is fist pumping after getting shot."

The gunman, who was identified as 20-year-old Thomas Matthew Crooks, had climbed a nearby rooftop with a direct line of sight to where Trump was speaking.

He was subsequently shot and killed by a Secret Service agent.

As blood dripped down his face, Trump raised a fist and shouted "fight, fight, fight" to his supporters, an image that had gone viral on social media.