Tempe, Arizona - President Joe Biden launched a blistering personal attack on "extremist" Donald Trump Thursday, accusing his likely 2024 challenger of plotting to subvert the US constitution if he wins a return to power next year.

The Democrat said in a speech in the battleground state of Arizona that Republican frontrunner Trump was driven by "vengeance and vindictiveness," and urged Americans to stand up for democracy before it was too late.

The speech came as House Republicans launched impeachment inquiry hearings into the 80-year-old president, based on unproven allegations that he lied about his son Hunter’s business dealings.

"We should all remember: democracies don't have to die at 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."

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In this speech, Biden took off the gloves after months in which he has largely steered clear of any direct comment on the multiple criminal indictments against his hard-right populist predecessor.

Biden has sought to avoid any appearance of interference in the cases stemming from Trump's efforts to overturn his 2020 election loss – culminating in the attack by a mob of his supporters on the US Capitol on January 6, 2021.

But in Arizona, Biden took the unusual step of addressing Trump by name, accusing him and his followers of attacking the free press and the rule of law and planning to gut US institutions if he wins a second term.

"Trump says the constitution gave him 'the right to do what he wants,'" said Biden, referring to statements 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."

In a dark warning, Biden said there was "something dangerous happening in America" and that the Republican Party was "driven and intimidated" by Trump's MAGA (Make America Great Again) fringe.

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He criticized Republicans for failing to speak up after Trump recently accused America's top military officer of treason, and lashed out at hardline Trump allies for pushing the United States towards a government shutdown this weekend.

The president sounded a still more personal note when he recalled how Trump had reportedly referred to service members captured or killed in action as "suckers and losers."

A fired-up Biden asked if that referred to John McCain, the late Arizona senator and Vietnam War icon whose legacy the speech honored – or Biden's own son Beau, who served in Iraq and died of brain cancer in 2015 aged 46.

"Was he (Beau) a sucker for volunteering to serve his country?" asked Biden.

Biden also touted his bipartisan credentials by hailing the legacy of his frequent political opponent and close friend McCain, who was an outspoken critic of Trump.

The feeling was mutual, with Trump mocking McCain in 2015, criticizing the fact that he had been taken prisoner in Vietnam.

Biden described McCain, who died in 2018, also of brain cancer, as a "proud Republican who put country first."

The president was interrupted at one point by a climate activist, before quipping to the audience: "Democracy is never easy – as was just demonstrated."

The bitter political divide that Biden lamented in his speech was underscored by the Republican impeachment inquiry hearings that began in Washington shortly before he spoke.

Republicans have accused Biden of trading on the power of his office when he was vice president under Barack Obama to help his younger son Hunter secure lucrative foreign business deals, and of benefiting personally from alleged corruption.

Democrats have dismissed the hearings as a stunt, but they will add to the pressure on Biden as he battles poor approval ratings and polls showing him neck-and-neck with Trump.

The current and former presidents were in campaign mode this week despite the election being more than a year away, with both men making visits to the swing state of Michigan to speak to autoworkers.