Erie, Pennsylvania - Donald Trump on Sunday used a speech in Pennsylvania to urge a "violent" crackdown on crime in his most extreme proposal yet.

The Republican former president and current candidate, who held a similar rally in swing state Wisconsin on Saturday, reprised his dark, racially charged message about an America crumbling under "invasion" by violent migrants and other criminals.

Recounting isolated – but widely publicized – incidents of thieves staging brazen daylight robberies of shops in major cities, Trump got a loud cheer when he said police should become "extraordinarily rough."

Those who commit crimes, he said, "have to be taught" and this could be done "if you had one really violent day."

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"One rough hour – and I mean real rough – the word would get out and it would end immediately," Trump said.

"The police aren't allowed to do their job" because "the liberal left won't let them."

Kamala Harris herself continued to tack right on the issue of immigration, citing supposed "serious problems" with so-called border security at a rally in Las Vegas.

"As president, I will double the resources for the Department of Justice to go after the transnational cartels," Harris said.

"We know Donald Trump won't solve them. When he was president, he did nothing to fix our immigration system," she added, calling for comprehensive reform but without offering details.

The vice president has instead repeatedly promised to revive a stalled bipartisan bill that contains the most draconian restrictions on immigration and the right to asylum in a generation.