Pittsburgh, Pennsylvania - Kamala Harris blasted Donald Trump as the "biggest loser" on the economy and a friend of billionaires Wednesday as the election rivals laid out competing plans on the top issue for many US voters.

In a speech on the economy and then again in her first major solo interview, the Democrat warned that Trump's plans to bring back huge tariffs on imports would hurt middle class Americans in their wallets.

Trump, for his part, doubled down on his protectionist vision – but spent as much time on threatening to blow Iran to "smithereens" after US intelligence warned of threats from Tehran against his life.

The vice president and the Republican are neck-and-neck in the polls and are both reaching out to undecided voters on key issues like the economy with less than six weeks until election day.

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Harris vowed to "chart a new way forward" in a speech in Pittsburgh, an industrial city in the critical swing state of Pennsylvania, with her rhetoric focusing on lowering prices for Americans.

"For Donald Trump, our economy works best if it works for those who own the big skyscrapers. Not those who actually build them. Not those who wire them. Not those who mop the floors," she said.

Nearly 200,000 factory jobs moved abroad during Trump's time in the White House, Harris said, "making Trump one of the biggest losers ever on manufacturing."

In her interview with MSNBC, Harris also slammed the tariff plans that Trump has laid out over the past two days, which would be a return to the policies of the Republican's first term in office.

"You don't just throw around the idea of tariffs across the board," said Harris. "He's just not very serious."

The interview was Harris's first on her own since replacing US President Joe Biden as the Democratic nominee in July. She gave a joint interview with running mate Tim Walz in August.

Trump's campaign responded by calling her speech was "full of lies" and that she had already had three and a half years as part of the Biden administration to tackle problems like low prices.