His comments come as the rival candidates clash over unrest in Portland, where a man was shot dead.
US presidential candidate Joe Biden has accused Donald Trump of being a “weak” and “toxic” leader who has “fomented” violence in the country.
Speaking in the battleground state of Pennsylvania, Mr Biden said the US was facing multiple crises, which “under Donald Trump, keep multiplying”.
The Democrat’s comments came amid sharp tensions between the rival candidates over unrest in US cities.
Mr Trump has made “law and order” a major theme of his campaign.
The two candidates have in recent days been trading insults over clashes in Portland, Oregon.
On Monday, police identified the man who was fatally shot during Saturday’s unrest as Aaron Danielson. His death was mourned by the far-right group Patriot Prayer, which described him as “a good friend and a supporter”.
Police have not specified whether the shooting was directly linked to the clashes between a pro-Trump rally and Black Lives Matter protesters in the city.
However, media reports say a man who calls himself an anti-fascist is being investigated over Danielson’s death.
Portland has become a flashpoint for demonstrations against police brutality and racism since the police killing of African-American man George Floyd in Minneapolis on 25 May triggered a wave of national and international outrage.
Mr Trump is set to visit the city of Kenosha, Wisconsin, on Tuesday amid anger there over the police shooting of black man Jacob Blake.
A teenager has been charged with killing two protesters during the unrest last week. His lawyers said he would fight the charges on the grounds of self-defence.
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