With House Republicans launching an impeachment inquiry against him, United States President Joe Biden went into offensive on Thursday with a chilling warning to Americans planning to elect Donald Trump in next year's presidential election.

In a speech in the battleground state of Arizona Thursday, the 80-year-old Biden accused the Republican primary frontrunner 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'," Biden said, 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."

The reelectionist president rallied Americans who are silent to stand up for democracy before it was too late.

Biden also turned the table on Trump for the former president's remark that US soldiers captured or killed in action as "suckers and losers."

"Was he a sucker for volunteering to serve his country?" Biden asked, referring to his son Beau, who served in Iraq and died of brain cancer in 2015 aged 46, and the late Arizona senator and Vietnam war hero John McCain.

Biden and Trump were in early campaign mode this week with both men visiting the swing state of Michigan to speak to auto workers.

Conservatives 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.

WITH AFP