Hillary Clinton
Hillary Clinton suggested Bernie Sanders was part of the 'establishment' after more than 20 years in Congress.Reuters

Democratic presidential frontrunner Hillary Clinton is not about to lose the Iowa caucus to rival Bernie Sanders without a fight. Clinton came out swinging during a 21 January interview with CNN's Wolf Blitzer on The Situation Room, by suggesting the Vermont senator was part of the "establishment" because he had served in Washington for longer than she has.

Sanders has accused Planned Parenthood and the Human Rights Campaign of only supporting Clinton because they are part of the "establishment," which Clinton said does not make sense to her. "I don't really understand what he means by that. These are two of the really great human rights, progressive organisations in our country," she said.

"I just don't understand what that means. He's been in Congress, he's been elected to office a lot longer than I have," she said. "He's been in the Congress for 25 [years]. And so I'll let your viewers make their own judgment," Clinton added, noting she only served in the Senate for eight years.

It should be noted that Clinton was also the first lady to President Bill Clinton for eight years and then served as President Barack Obama's secretary of state during his first four years in office.

According to CNN, the former secretary of state also attacked Sanders's experience to lead country, taking particular aim at his debate suggestion that the US should warm relations with Iran. "I think it would be a mistake to offer normalised relations," Clinton said. "I engaged in long, patient diplomacy to put together the sanctions on Iran that finally brought them to the negotiation table and actually began the preliminary negotiations that the President and Secretary Kerry continued to completion."

US election 2016: Guide to the Democrat candidatesIBTimes UK

Clinton's comments come on the heels of a new CNN/ORC poll that revealed Sanders is leading Clinton among Democratic likely Iowa caucus voters, 51% to 43%. The results mark a swift flip from December 2015, when Clinton led 54% to Sanders's 36%. A third long-shot candidate, Martin O'Malley, has remained steady at 4% for the last two months.

On his side, Sanders has upped his campaigning in Iowa and New Hampshire less than two weeks before the 1 February Iowa Caucus. ABC News reported the Sanders campaign has released three new adverts in two days.

The first advert focuses on oil pipelines and the environment, while the second focuses on the senator's vote against the Iraq War. The third, which is more of a teaser than campaign advert, shows crowds of people out seeing Sanders and donating to his campaign as Simon and Garfunkel's America plays in the background.