US President Joe Biden said "there is something very dangerous happening in America" as Donald Trump leads his rivals in the race to be named the Republican presidential nominee in 2024.

In an address on Thursday, Biden accused Trump's supporters of attacking the rule of law, the free press and democracy.

"We should all remember: democracies don't have to die the end of a rifle," Biden said. "They can die when people are silent, when they fail to stand up or condemn threats to democracy, when people are willing to give away that which is most precious to them because they feel frustrated, disillusioned, tired, alienated."

Biden said, "There is no question that today's Republican Party is driven and intimidated by MAGA extremists," referring to the acronym for the former president's slogan to "Make America Great Again."

The speech was Biden's fourth in a series discussing what he sees as challenges to democracy.

The location gave the speech added effect, as it was an event in Arizona to honor Biden's late friend, political rival and fierce Trump critic from within the Republican Party, Senator John McCain.

Trump driven by 'vengeance and vindictiveness'

Biden typically refers to Trump as his "predecessor" or "the last guy."

But during his speech on Thursday, he directly condemned the Republican front-runner with some of his toughest words to date.

"Trump says the constitution gave him 'the right to do what he wants,'" said Biden, referring to statements made by Trump about how he saw his powers in office.

"I've never even heard a president say that in jest — not guided by the constitution or by common service and decency towards our fellow Americans, but by vengeance and vindictiveness."

Trump skipped the second Republican primary debate on Wednesday. He currently leads in the polls by a margin as wide as all his opponents' polling numbers combined.