Hillary Clinton has attacked Vermont Senator Bernie Sanders and his campaign again by calling it all "just baloney".

The former first lady and senator appeared on Good Morning America to discuss her new four-part Hulu docuseries called "Hillary".

During the interview, Mrs Clinton doubled down on her criticisms against the senator and his campaign, saying her opinions about him since they ran against each other in 2016 for the Democratic Party nominee has not changed.

"That was my authentic opinion then; it's my authentic opinion now," she said.

The moment during the 2016 campaign being referred to was when Mrs Clinton called his Sanders' efforts "just baloney" when getting things done in Congress.

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"He was in Congress for years. He had one senator support him. Nobody likes him, nobody wants to work with him, he got nothing done," Clinton said in the documentary. "He was a career politician. It's all just baloney and I feel so bad that people got sucked into it."

These opinions, she reiterated, held true today.

Mrs Clinton was also asked to respond to Mr Sanders' shifting views about the delegation process and how much is needed to pick the nominee. The presidential candidate now says the person leading in number of delegates at the 2020 Democratic Convention should be the nominee, a position he differed from in 2016.

"My reaction is let's follow the rules," the former first lady said. "We've got rules. We had rules last time and we have rules this time. I think it's always a good idea to follow the rules. Everybody knew what they were when you got into it."

Her criticisms about Mr Sanders come on Super Tuesday with the presidential candidate currently the front-runner in the race and expected to pick up a large number of delegates from the 14 states.

While Mr Sanders is currently in the lead, the former first lady said much of the race was still to go before deciding the Democratic nominee.

"Only four or five per cent of the people whose views are going to be voted on have actually had a chance to express themselves so there's a long way to go," Mrs Clinton said.

"Today obviously is a big day," she added. "I'm just watching and hoping that we nominate whoever is the strongest candidate to take out the current incumbent. That's the only thing that really matters at the end of the day."

Former Vice President Joe Biden could experience a bump in the polls among moderate voters after both Pete Buttigieg and Amy Klobuchar dropped out of the race to then put their endorsement behind him.

When commenting on the dramatic shift in the race, Mr Sanders said the "establishment wants me to lose".