Donald Trump accused rival Hillary Clinton of being against the police as he condemned the violence that followed a shooting by officers.
The billionaire tycoon visited Wisconsin, the latest state to be hit by riots after the fatal shooting by police of a black man.
Milwaukee's Sherman Park neighbourhood erupted in chaos on Saturday night after a black suspect was fatally shot by a black Milwaukee police officer.
Businesses burned, gunshots rang out and police in riot gear were pelted with rocks and other objects. The violence continued, to a lesser degree, on Sunday night.
On Tuesday, Republican candidate Mr Trump seized on the riots, accusing Mrs Clinton of sympathising with protesters, who have complained of systemic racism and inequality at the hands of police.
Mr Trump, who is lagging behind in the polls, said: "Our opponent Hillary would rather protect the offender than the victim."
He added: "The riots and destruction that have taken place in Milwaukee is an assault on the right of all citizens to live in security and to live in peace."
Mr Trump began his visit to the city with a meeting with local law enforcement officers at the Milwaukee County War Memorial Centre on Lake Michigan.
Among those present were Milwaukee County Sheriff David Clarke, who penned an article on Monday blaming liberal Democrats and the media for the unrest that has rocked the city.
He told Fox News that the shooting in Milwaukee may have occurred because the officer had a gun to his head.
"Who can have a problem with that?" Mr Trump said. "If it is true, then people shouldn't be rioting."
Mr Trump, who has so far proven deeply unpopular with black voters, made a direct appeal to them: "I'm asking for the vote of every African American citizen struggling in our country today who wants a different and much better future."
He claimed Mrs Clinton and other Democrats have "betrayed the African American community" and pandered for votes.
"We reject the bigotry of Hillary Clinton, which panders to and talks down to communities of colour and sees them only as votes - that's all they care about," Mr Trump said.
Clinton campaign spokeswoman Jennifer Palmieri responded by accusing Mr Trump of being the bigot instead.
She said: "With each passing Trump attack, it becomes clearer that his strategy is just to say about Hillary Clinton what's true of himself.
"When people started saying he was temperamentally unfit, he called Hillary the same. When his ties to the Kremlin came under scrutiny, he absurdly claimed that Hillary was the one who was too close to Putin. Now he's accusing her of bigoted remarks -- We think the American people will know which candidate is guilty of the charge."
Mrs Clinton had said Monday the Milwaukee protests showed that the nation had "urgent work to do to rebuild trust between police and communities" and said "everyone should have respect for the law and be respected by the law".
Mr Trump has stoked tensions during his campaign. He has accused Mexico of sending rapists across the border, has feuded with the Muslim-American parents of an Army captain killed in Iraq and has proposed to suspend immigration by Muslims.
While polls have shown Mrs Clinton building a lead, Democrats are fearful that a depressed voter turnout might diminish support among the minority, young and female voters who powered Barack Obama to two victories.