 -- Vermont Sen. Bernie Sanders called Republican frontrunner Donald Trump a "pathological liar" today on ABC's "This Week," saying "much of what he says are lies or gross distortions of reality."

"I say this straight-forwardly, I think you have a pathological liar there," he said.

Sanders disputed Trump’s claim there were Muslims celebrating in New Jersey after the September 11th attacks. He argued there has been no video evidence.

"Either it is true or it is not true," Sanders said. "Nobody has seen a tape of thousands of people celebrating the destruction of the Twin Towers in New Jersey. It doesn't exist, and [Trump] keeps claiming it. That's called pathological lying."

Sanders argued Trump's campaign was resonating with Republican voters because his solutions to problems were "easy." He said it was harder to have "rational" conversations about how to rebuild the American middle class. He also called Trump a "demagogue."

“What somebody like a Donald Trump is doing is playing on the fears and anxieties of the American people,” he added. "You defeat it by going out to the working people in this country and making it clear that if we are going to create an economy that works for our kids, that works for working families, we have got to do it together. We have to understand the real causes and the real causes are the greed of corporate America, the greed of Wall Street, not some poor Mexican who is in this country trying to live on $7 an hour."