Democratic White House hopeful Joe Biden on Monday castigated President Donald Trump as a "weak" and morally deficient leader who has sown chaos and fomented the violence that has recently gripped US cities.

"This president long ago forfeited any moral leadership in this country. He can't stop the violence -- because for years he has fomented it," the Democratic presidential nominee will say in a speech in Pittsburgh, according to excerpts released by his campaign.

"He may believe mouthing the words 'law and order' makes him strong, but his failure to call on his own supporters to stop acting as an armed militia in this country shows you how weak he is," Biden will say.

Two US cities in particular -- Kenosha, Wisconsin and Portland, Oregon -- have been battered by fierce protests and accompanying deadly violence, with supporters of Trump and Biden facing off in dangerous encounters.

Biden is expected to address the multitude of crises currently affecting the United States, including the coronavirus pandemic that has now left more than 183,000 Americans dead, the resulting "economic devastation," and violent incidents by "emboldened" white nationalists.

He will also present a counter argument to Trump's messaging during last week's Republican National Convention, when he warned that people would not be safe "in Joe Biden's America."

"Does anyone believe there will be less violence in America if Donald Trump is reelected?" Biden will say in his speech. 

The common thread to the multiple crises is "an incumbent president who makes things worse, not better. An incumbent president who sows chaos rather than providing order," Biden will say.

Trump has repeatedly tweeted "LAW & ORDER!" in recent days, accused Portland's "radical left" Democratic mayor of losing control of his city, and threatened to "go in" with federal forces to restore order.