The Vermont senator said he did not feel good about using harsh words to describe the President, but felt, given Mr Trump's behaviour, his terminology was accurate.
Speaking to CNN presenter Erin Burnett, he said that although he is considerably to the left of most of his fellow senators, he has never “just attacked people because their views are different than mine”.
But Mr Sanders said Mr Trump, who he described as “a total hypocrite” and a "pathological liar", had put everyone in “a new era”.
“I use the term 'delusional' with regard to Trump when he said there were 3 to 5 million 'illegals' who voted in the last election ... That is delusional,” Mr Sanders said.
“Nobody in the world believes that is the case. There is zero evidence to back it up. But he makes that statement. So I think the word 'delusional' is correct.”
Mr Sanders, who challenged Hillary Clinton for the Democrat leadership, also criticised Mr Trump for attacks on the media and the judiciary.
He said Mr Trump did not grasp the Constitution, which as President he swore to uphold.
“Clearly, we have a President who does not understand what our Constitution is about, what democracy is about. And I think there is a fear in this country of this nation under Trump moving into a more authoritarian mode,” Mr Sanders said.
Mr Sanders said he did not feel good using such harsh terms, but "ordinary people, working people, middle class people, are understanding that this guy is not delivering what he promised".
Although he disagrees with them, he said he considers a lot of Republicans decent, honest people — just not the President.
“I disagreed with George Bush all the time — never called him a pathological liar, because he was not. Just a conservative president. But this guy lies all of the time,” Mr Sanders said of Mr Trump.
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