US Vice President Mike Pence has cast the re-election of President Donald Trump as critical to preserving America’s safety and economic viability while cautioning Democrat Joe Biden will set the country on a path to socialism and decline.

Amid widening protests over the police shooting of Jacob Blake, a black man, in Wisconsin, Pence and other Republicans at their national convention described the November 3 contest between Mr Trump and Mr Biden as a choice between “law and order” and lawlessness.

“The hard truth is you won’t be safe in Joe Biden’s America,” Pence told the crowd seated on a lawn at historic Fort McHenry in Baltimore.

Accompanied by first lady Melania Trump, Mr Trump joined Pence on stage after the speech as the crowd chanted: “Four more years.”

Police in Kenosha, Wisconsin, shot Mr Blake, 29, multiple times in the back at close range on Sunday, reigniting protests against racism and police brutality that erupted across the US earlier in the year.

During a third night of unrest on Tuesday, three people were shot, two fatally.

The state’s lieutenant governor said the white teenager arrested on homicide charges was apparently a militia member who sought to kill innocent protesters.

Ahead of Mr Pence’s speech, Mr Trump said he would send federal law enforcement to Kenosha by agreement with the state’s governor.

“Let me be clear: the violence must stop – whether in Minneapolis, Portland or Kenosha,” said Mr Pence, who did not mention Mr Blake.

“We will have law and order on the streets of this country for every American of every race and creed and colour.”

Mr Biden said earlier he had spoken with Mr Blake’s family and, like the family, called for an end to the violence.

“Protesting brutality is a right and absolutely necessary, but burning down communities is not protest. It’s needless violence,” Mr Biden said in a video posted by his campaign.

Mr Pence repeated the unfounded charge that Biden supports liberal activist calls to “defund” the police.

Mr Biden rejects that approach and has promised instead to invest $US300 million in grants to hire more diverse officers and train them to develop better relationships with communities.

Mr Trump has intensified his “law and order” campaign theme as polls have shown voters give him poor grades for his handling of the coronavirus pandemic, which has killed more than 179,000 Americans.

A former Indiana governor and congressman and a conservative Christian, Mr Pence has served as a key connector between Mr Trump and evangelical voters, an influential part of the Republican political base.

Mr Pence sought to reshape the narrative around the economy, casting the millions of jobs lost to the pandemic as a temporary setback.

He said the US was on track to have the world’s first safe, effective coronavirus vaccine by the end of this year.

Mr Pence accused Mr Biden of being a stalking horse for the radical left.

“Last week, Joe Biden said democracy is on the ballot, and the truth is our economic recovery is on the ballot, law and order are on the ballot,” Pence said.

-AAP