US Vice President and Democratic candidate Kamala Harris warned Monday against the consequences if her Republican rival Donald Trump is elected president.

"I think it's very important that we acknowledge, and I have said publicly, that Donald Trump is an unserious man, and the consequences of him ever being president of the United States again are brutally serious," Harris told supporters in Wisconsin.

The world is watching the Nov. 5 presidential election, and the US's allies are worried, she added.

Stressing that Trump is "easily manipulated by flattery," Harris reminded the supporters what he reportedly did during the COVID pandemic.

"You remember what that was during that time. He secretly sent COVID tests to (Russian President) Vladimir Putin for his personal use," she said.

Turning to Trump's repeated remarks on the Russia-Ukraine war, Harris said that "on the issue of Ukraine, he says, 'Oh, well, I'd solve that in a day.' Well, I don't think we as Americans think that the president of the United States should solve an issue like that through surrender and understand that's what would happen."

"Vladimir Putin would be sitting in Kyiv if Donald Trump were president," she underscored, adding that Trump "wants to please somebody that he considers to be a strong man who he admires."

Last week, Trump said he had a "very good" relationship with Putin. Declining to comment on whether he has spoken to the Russian leader since leaving office in 2021, he suggested that if he had, "it’s a smart thing."

In a book, journalist Bob Woodward cited an unnamed aide saying that Trump had spoken with Putin as many as seven times since leaving the White House.

An official from the Biden administration said they were unaware of reported Trump-Putin calls, but US National Security Adviser Jake Sullivan told reporters that "obviously that would raise red flags if it were true."