Donald Trump has been branded a pathological liar in a scathing attack by Bernie Sanders, who has accused the President of undermining democracy.

The senator, who challenged Hillary Clinton for the Democratic presidential nomination last year, said Trump lies 'all the time'.

During the extraordinary attack, Sanders called on Americans to resist the President's assault on the media, judiciary and election process.

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The senator, who challenged Hillary Clinton for the Democratic presidential nomination last year, said Trump lies 'all the time'

It comes days after he accused the Republican of having a 'complete disregard for reality'

Speaking to The Guardian, the Vermont senator said: 'Trump lies all of the time and I think that is not an accident, there is a reason for that. He lies in order to undermine the foundations of American democracy.'

It comes days after he accused the Republican of having a 'complete disregard for reality'.

On Tuesday Sanders posted on Twitter: 'We have a president who either lies intentionally or, even more frighteningly, does not know the difference between lies and truth.'

And he continued: 'I strongly believe in civil political discourse. But how does one respond to a president who has complete disregard for reality?'

It comes days after Sanders accused the Republican of having a 'complete disregard for reality'

On Tuesday Sanders posted on Twitter that Trump 'does not know the difference between lies and truth'

He also said the White House incumbant has a 'complete disregard for reality'

In his Guardian interview, he accused Trump of trying to present himself as 'the only person in America who gets it right' and compared former president George W. Bush favourably with the current White House incumbent.

Sanders said: 'George Bush was a very conservative president, I opposed him every single day. But George Bush did not operate outside of mainstream American political values.'

In his first 50 days as President, Trump has attacked a 'so-called' judge for opposing his travel ban, branded sections of the media 'enemies of the people' and accused his predecessor, Barack Obama, of ordering a wiretap on Trump Tower.

The President is yet to produce any evidence to substantiate the latter claim.

Trump accused Barack Obama with wiretapping Trump Tower, but is yet to produce any evidence to substantiate the claim

Sanders said anti-Trump protests, such as this one in San Francisco, show Americans will resist the President's authoritarianism

Sanders reached out to Republicans to help resist the President's authoritarianism, stating: 'It is incumbent upon them, in this moment in history, to stand up and say that what Trump is doing is not what the United States is about, it’s not what our constitution is about. They have got to join us in resistance.'

Widespread protests across America during the first days of Trump's presidency are indicative of a strong resistance movement, Sanders said.

He told The Guardian that a 'grassroots resistance' is the only way to defeat what he described as Trump's moves toward authoritarianism.

Sanders also called for clarity over Russia's influence on the Trump administration, stating: 'We need to know what kind of influence the Russian oligarchy has over Trump. Many people are astounded.'

The former Democratic presidential hopeful has been a vocal critic of Trump since he took office.

Last month, when the President addressed a joint session of Congress having invited the families of people killed by immigrants, Sanders accused Trump of 'stirring up fear and hatred against immigrants' and 'trying to divide our nation'.

Sanders wrote on Facebook: 'The murder of anyone is a tragedy, and our hearts go out to all families who lose a loved one to violence.

'But let's be clear about what Donald Trump is doing tonight in inviting family members who saw a loved one murdered by an undocumented immigrant.

'He is stirring up fear and hatred against immigrants and trying to divide our nation. That is his political strategy and we must not allow him to get away with it.'