US Senator and former presidential candidate Bernie Sanders has launched a withering attack on President Donald Trump, accusing him of violating the US Constitution and repeatedly lying to the nation.

In an interview with The Guardian newspaper on Friday, Sanders said Trump's policies show he is driving the country toward “authoritarianism.”

Sanders, the independent senator from Vermont who ran against Hillary Clinton for the Democratic presidential nomination in 2016, gave a bleak assessment of the new Trump administration and its intentions.

He warned that Trump’s most controversial comments against the media, judiciary and other public institutions amounted to an assault on democracy.

“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.”

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Speaking in his Senate office in Washington, DC, Sanders also harshly criticized Trump’s economic policies, citing the president’s tax breaks for billionaires and devastating cuts to domestic programs that impact the middle class.

Trump’s objective was to “end up as the leader of a nation which has moved in a significant degree toward authoritarianism,” he said. “The only way to defeat that trend is for massive grassroots resistance, and clearly we are seeing that right now.”

Sanders, however, expressed satisfaction that Trump’s lies and policies have given rise to national resistance, pointing to 150 anti-Trump rallies in one recent weekend alone.

“You are seeing a very active progressive movement. Our Revolution – a group which came out of my campaign – other groups, the spontaneous Women’s March, that’s all an indication of the willingness of the American people to fight back for democracy,” he said.

The Vermont senator appealed to his Republican colleagues in Congress to join him in his resistance. “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.”