WASHINGTON - Republican US presidential candidate Donald Trump said on Tuesday the European Union would have to "pay a big price" for not buying enough American exports if he won the Nov 5 election.

"I'll tell you what, the European Union sounds so nice, so lovely, right? All the nice European little countries that get together," Trump said during a rally in the battleground state of Pennsylvania, after promising to pass the "Trump reciprocal trade act."

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"They don't take our cars. They don't take our farm products. They sell millions and millions of cars in the United States. No, no, no, they are going to have to pay a big price," he said.

Democrat Kamala Harris warned tens of thousands of people gathered in Washington at her biggest rally on Tuesday that her Republican opponent Donald Trump was seeking unchecked power as their tightening race for the presidency entered its final week.

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"This is someone who is unstable, obsessed with revenge, consumed with grievance and out for unchecked power," Harris said during what her campaign called her closing argument before a tightly contested Nov 5 election.