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 programme.

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 criticised 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."

When asked what drives Trump's support, she told the BBC that "people support him for different reasons" including feeling "overlooked" and "unseen" or that the "economy doesn't work for them".

"I think our problem is frankly we are not the most effective messengers," she said of the Democratic Party. "About what we see and what we’re trying to do to address these real and legitimate concerns that people have. I recognise and accept my share of the responsibility."

The expectation upon politicians to be entertaining or outrageous in a social media age makes it harder to "do the hard, boring work of actually getting things done," she added.

Polling suggests Trump is more trusted on the election's biggest issue, the economy.

The election will be held on 5 November and the new president will take office in January.

Polls are currently very tight in the seven states considered as crucial in the contest, with just one or two percentage points separating the two candidates.

Clinton, 76, was the first woman nominee for president from a major political party when she ran against Trump in 2016. Her husband, Bill Clinton, whom she married in 1975, was president from 1993 until 2001.