Donald Trump said Sunday it was divine intervention that helped him survive an assassination attempt during a campaign rally in Pennsylvania, and called on Americans to unite.

"It was God alone who prevented the unthinkable from happening," the former US president and White House hopeful said on social media, urging fellow Americans to unite in "not allowing Evil to Win."

Trump was hit in the ear in an assassination attempt by a gunman at a campaign rally Saturday, in a chaotic and shocking incident set to supercharge political tensions ahead of the US presidential election.

The 78-year-old former president was rushed off stage with blood streaked across his face after the shooting in Butler, Pennsylvania, while the shooter and a bystander were killed and two spectators were critically injured.

President Joe Biden, who is set to face Trump in November's deeply polarized presidential election, said there was "no place in America for this kind of violence."

"In this moment, it is more important than ever that we stand United, and show our True Character as Americans, remaining Strong and Determined, and not allowing Evil to Win," Trump wrote on his Truth Social platform Sunday morning, confirming that he would attend the Republican National Convention, which begins on Monday in Milwaukee, Wisconsin.