WASHINGTON — Hillary Clinton has told the BBC that Joe Biden was right to stand aside in the US presidential election after his stumbling debate performance against Donald Trump earlier this year.

"Once that debate happened, he could not recover and he did the right thing," she told Radio 4's Today program.

She also said Democrats had not been effective in telling Trump supporters what they were doing to address their economic concerns.

In 2016, the former US secretary of state was unexpectedly defeated by Trump, who now faces Democratic candidate Kamala Harris in November's election, which polls suggest is extremely tight.

She said she saw Joe Biden a week before the debate and saw no reason then why he should step down but that all changed.

Clinton also said the future of democracy is at stake in this election and called on Harris to "defeat Donald Trump to break the fever that he has caused in our political system".

"The two candidates have presented extremely different agendas for where they want to take our country," Clinton said in the BBC interview, which came as she promotes her new memoir.

Trump has rejected the notion that he is a threat and said the real threat comes from the Democratic Party.

The election, Clinton said, would have repercussions far beyond the US including "whether or not we continue supporting Ukraine, whether we can get some kind of workable resolution in the Middle East and so much else".

Trump has previously indicated that he would cut US aid for Ukraine. After meeting Ukrainian President Volodymyr Zelensky in New York on Friday, the former US president said he had "a very good relationship" with both Zelensky and his Russian counterpart Vladimir Putin.

“We both want to see a fair deal made," Trump said. The war, he added, "should stop and the president (Zelensky) wants it to stop, and I’m sure President Putin wants it to stop and that’s a good combination.”

Clinton, who served as secretary of state in the Obama administration from 2009 to 2013, also criticized Trump's promise to begin a mass deportation of illegal immigrants if elected.

"Let's start with one million,” his vice-presidential pick JD Vance said of the plan in August. “That's where Kamala Harris has failed. And then we can go from there.”

"He is going to have a military presence [in US cities] to achieve his goals," Clinton said on Monday. "If you look a certain way, if you talk a certain way, you will be subject to these Draconian measures." — BBC