Vermont Sen. Bernie Sanders opened up a new line of attack in the Democratic presidential primary on Wednesday, putting Hillary Clinton on the defensive over her liberal credentials just days after she eked a slim victory in the Iowa caucuses.

Sanders, who has a sizable lead in the upcoming New Hampshire primary, rattled off a list of issues where Clinton isn't in sync with the liberal wing of the party, including trade, Wall Street regulation, climate change, campaign finance and the 2002 authorization of the war in Iraq.

"I do not know any progressive who has a super PAC and takes $15 million US from Wall Street," Sanders said, during a candidate forum sponsored by CNN. "That's just not progressive."

Clinton moved quickly to defend her record, saying that under Sanders' criteria President Barack Obama, Vice President Joe Biden and even the deceased Minnesota Sen. Paul Wellstone, a champion of liberal causes, would not be considered progressive.

"I know where I stand," said Clinton. "But I don't think it helps for the senator to be making those kinds of comparisons because clearly we all share the same hopes and aspirations for our country."

'I do not know any progressive who has a super PAC and takes $15 million US from Wall Street.' - Bernie Sanders

She also pushed back on charges by Sanders and his allies that she cannot be trusted to regulate Wall Street because of the millions in speaking fees she made from the industry before announcing her presidential bid. An Associated Press analysis of public disclosure forms and records released by her campaign found that Clinton made $9 million from appearances sponsored by banks, insurance companies, hedge funds, private equity firms and real estate businesses.

Clinton said she was still deciding whether to run for president when she accepted the appearances

"I don't know," she said, when asked why she was paid such a high speaking fee. "That is what they offered."

The back-and-forth on progressive credentials was the latest example of tensions between Clinton and Sanders as the race nears the Feb. 9 New Hampshire primary. The Democratic rivals are expected to appear at a debate on Thursday night and both camps have quarreled over the timing and locations of three debates planned for later this spring.

Clinton has questioned Sanders' commitment to gun control and whether his proposal to create a universal health care system might endanger Obama's signature health care law. Sanders, meanwhile, casts Clinton as an establishment figure and an inconsistent champion of liberal causes such as the environment, trade and campaign finance reform.

Speaking at a town hall meeting in Derry, N.H., earlier in the day, the former secretary of state called Sanders attacks on her ideology a "low blow," before listing a series of liberal accomplishments that she described as progressive, including her work on expanding access to children's health insurance, advocating for women and gay people and pushing for gun control measures.

"We've been fighting the progressive fight and getting results for people for years," Clinton said. "I hope we keep it on the issues. Because if it's about our records, hey, I'm going to win by a landslide."

But Clinton's team clearly sees an opening in Sanders' comment. On Twitter, Clinton's top spokeswoman Jennifer Palmieri compared it to the moment in 2008 when Obama said during a debate that Clinton was "likable enough," which prompted criticism from Clinton supporters.

The attack came from a comment Clinton made at a campaign event in September, when she was describing tax cuts passed under former President George W. Bush and noted that she's occasionally been called a moderate. "I plead guilty," she told the crowd in Columbus, Ohio.

Clinton needs to tackle huge youth gap

Sanders cited her words in a Wednesday evening news conference in Concord, before noting that she has done some "progressive things" like advocating for children.

"This is not a low blow. There's nothing wrong with people who are moderates. Some of my best friends are moderates," he said. "All I was doing was repeating what she actually said."

Sanders' razor-thin loss in the Iowa caucuses Monday, and his formidable lead in New Hampshire polls, have heightened the possibility that the two remaining Democrats will be involved in a protracted fight for the nomination.

"We are in this until the convention," Sanders told reporters on Tuesday. He said the narrow Iowa outcome showed his campaign's ability to take on Clinton's vast political network and address doubts among voters about his electability.

Clinton acknowledged that she yet to win over broad swaths of the party, particularly younger voters. In Iowa, Sanders won 84 per cent of voters under age 30 and 58 per cent of those aged 30-44 according to entrance polls.

"I respect the fact that I have work to do," said Clinton. "They don't have to be for me, I will be for them."

Meanwhile, the 74-year-old Sanders saved some sharp barbs for Republican candidate Donald Trump, calling him out for his "bigoted" remarks and his position on climate change.

"Trump is, as you know, a well-known scientist, brilliant scientist," Sander said. "And he has concluded, after years of studying the issue, that climate change is a hoax brought to us by the Chinese.

"That shocked me because I thought he would have thought it was a hoax brought to us by the Mexicans or the Muslims."