US Democratic presidential candidate Hillary Clinton leads a campaign rally at Abraham Lincoln High School in Council Bluffs, Iowa January 31, 2016.  [Photo/Agencies]

Sanders refused to be drawn on the simmering controversy over Clinton's use of a private email server while secretary of state, telling ABC's This Week he would not "politicize" questions about whether Clinton may have compromised classified information.

The issue flared up again on Friday, when the US government said 22 emails containing material demanding one of the highest levels of classification had been found on the private server.

Clinton sought on Sunday to reiterate her position on the issue, telling ABC "there is no classified marked information on those emails sent or received by me".

Her solution, she said, would be to "release the emails, let the public see, and move on".

"I take classified information very seriously," she said. "You can't take information off the classified system onto the unclassified system, no matter what that is."

In response to comments made in the Guardian by former secretary of labor Robert Reich, naming Sanders as the candidate to combat inequality, Clinton argued that as the campaign continued she would bring in more people who felt like they were on the fringes of society.

Clinton predicted she would eventually win over doubters, and said: "When I was secretary of state even the Republicans said I was doing a good job."

She also, however, alluded to ferocious attacks from Republicans over her record as the top US diplomat under Obama, saying "this is very much like Benghazi".

Last year, Clinton survived relatively unscathed an 11-hour congressional grilling regarding the 2012 attack on a US facility in the Libyan city in which four Americans including the ambassador were killed.

With campaigning in Iowa reaching a crescendo, focus was turning to whether the two outsider candidates at the top end of the polls, Sanders and Donald Trump, can now turn voter anger into voter action.

On the Republican side, Iowa Republican party chairman Jeff Kaufman said he anticipated turnout easily surpassing existing records. On Saturday, Trump told a large crowd in Dubuque that if they didn't show up to caucus tomorrow the effort would be wasted.

"I don't care what it is," he said. "If you don't get out, we're wasting time."

On Sunday, Trump rejected accusations leveled by Senator Ted Cruz, his closest challenger in Iowa, that he skipped this week's Fox News debate in order to hide his liberal past.

Speaking to Fox News Sunday, Trump also defended past contributions to the Clinton Foundation that records suggest reached as much as $250,000.

Trump said he understood at the time that the Clintons were engaged with reconstruction in Haiti, not renting private aircraft.

"I was a businessman and it was my obligation to get along with everyone," he said.

Cruz, meanwhile, responded to Trump's needling over undeclared loans, saying the candidate who is five points ahead in the polls was unnerved.

"Donald looks rattled," Cruz said, also on Fox News Sunday. "That why he's insulting me everyday. It's the height of chutzpah."