Jill Biden says Donald Trump's now-infamous phone call to Ukrainian president Volodymyr Zelensky proves he is 'afraid' of going up against her husband, Joe, in 2020. 

The former Second Lady of the United States, 68, made the claim during an interview with MSNBC on Saturday. 

Addressing the phone call - which is now at the heart of the impeachment probe - she stated: 'When Joe and I decided to run, we knew it was going to be tough.. but we could never have imagined that it would turn into Donald Trump asking a foreign government to get involved in our elections.

'I think, you know Donald Trump has shown us who he is and this has been a real distraction, and I think it just proves that he's afraid to run against my husband'. 

Trump has vehemently denied he was trying to dig for dirt on Joe and his family  during his call with Zelensky, in which he was asked the Ukrainian president to look into the firing of a former prosecutor. 

The prosecutor was allegedly fired while trying to investigate Burisma - a Ukranian gas company where Joe's son, Hunter Biden, served on the board.  

Hunter was reportedly paid up to $50,000 a month for the plum position, despite having no prior experience in the gas industry and failing to speak the local language. 

Many have seen Hunter's position on the board of the Ukrainian company as a conflict of interest, given that his father was serving as vice president at the time.

However, during her MSNBC interview on Saturday, Jill staunchly defended her son, stating:  'I know my son's character. Hunter did nothing wrong, and that's the bottom line.'

She did not reference Hunter's messy current paternity case with Arkansas-based former stripper, Lunden Roberts. 

Lunden, 28, filed suit against Hunter, 49, in May, seeking child support for their 16-month-old child - a child he denied was his until a DNA test proved otherwise. 

She also made no mention of reports that Hunter is also expecting his fifth child with new wife, Melissa Cohen, whom he wed after just 10 days.  

However, Jill did speak with MSNBC about a litany of complaints critics have directed towards her husband and his campaign. 

Last week, Joe called a New Hampshire primary voter a 'damn liar' during a campaign stop, after the man accused the former vice president of sending his son to Ukraine to 'sell access' to the presidency. 

'You fight for your kid, Joe's going to stand up to bullies, bullies like Donald Trump,' Jill said, defending Joe's behaviour, which many saw as aggressive. 

'It showed his strength, his tenacity, the fire in the belly!'

She continued: 'You can never say anything against somebody's child. That's bullying, look at what the president did this week with that 16-year-old girl Greta [Thunberg]. You can't attack children!'

She also refuted claims that at the age of 77 Joe is too old for the presidency. 

'Most of them [the Democratic frontrunners] are in their 70s... Just come and travel with me, travel with Joe. He has a lot of energy. He's always been like that!'

She also told MSNBC she was surprised by reports that Joe told staffers that he would consider stepping down after just one term as president.  

A senior adviser reportedly told Politico last week that 'if Joe is elected, he's going to be 82 years old in four years and he won't be running for reelection'.  

The source reportedly said: 'If Joe chose a young and diverse running mate and Cabinet, the adviser argues he could offer himself to the Democratic electorate as the candidate to help usher in the next generation of Democratic political leaders.

'This makes Joe a good transition figure.' 

His campaign pushed back strongly against those reports. 

'Lots of chatter out there on this so just want to be crystal clear: this is not a conversation our campaign is having and not something VP Biden is thinking about,' the Biden Deputy Campaign Manager Kate Bedingfield tweeted Wednesday.