Republican US presidential candidate Donald Trump has campaigned at a Pennsylvania site where he was shot during a rally in July.

On Saturday, the former US president returned to Butler in the swing state where a bullet grazed his right ear while he was speaking at a podium.

Trump said that for 16 harrowing seconds during the gunfire, time stopped but the villain did not succeed in his goal.

He said, "He did not shake our unyielding resolve to save America from evils of poverty, hatred and destruction."

He then chanted, "Fight. Fight. Fight. Fight," with the audience, reminiscent of the pose he took immediately after he was shot.

With a month to go before the Presidential election, Trump is apparently trying to project an image of strength to win more votes in a state that can determine the outcome of the race.

Billionaire entrepreneur Elon Musk, who had earlier endorsed Trump, took the stage at the rally. He said it is important to register to vote. He went onto say, "Get everyone you know and everyone you don't know, drag them to register to vote."

Trump supporters waited in long lines with some arriving more than 10 hours before Trump's speech.

There was large police presence around the venue. Snipers were deployed on the rooftops of nearby buildings.