Joe Biden claims he's getting 'beat up' in the Democratic presidential primary 'because I'm not the socialist' - unlike rival Bernie Sanders.

The former vice president made his claim at a fundraiser in Florida on Monday night at a time when the socialist label has been thrown around as an insult in the primary process.

Both Sanders and Elizabeth Warren have worked to capture the left-flank of the Democratic Party - in contrast to Biden's more moderate roots.

Joe Biden claims he's getting 'beat up' in the Democratic presidential primary 'because I'm not the socialist'

Biden has referred to Bernie Sanders as a socialist and Elizabeth Warren is also looking to capture the left wing of the Democratic Party

Biden criticized Sanders as a socialist in the September Democratic primary debate when the two men tangled over health care.

'For a socialist you got a lot more confidence in corporate America than I do,' Biden told him.

The socialist label has gotten tossed around in the Democratic presidential primary field, particularly in the health care debate and about which candidates support Medicare For All, a universal health care plan, and the Green New Deal, a climate proposal led by Alexandria Ocasio-Cortez.

Sanders and Warren have championed both proposals.

And Sanders is a self-described democratic socialist.

'What I am trying to do, in many ways, is pick up where Franklin Delano Roosevelt left off,' he told CNBC in an interview that aired Tuesday.

He also claimed to be more liberal than Warren, pointing out she describes herself as a capitalist.

'It’s not just more regulation. It’s about involving millions of people, working people, young people, people who believe in justice, in the political process, to tell the corporate elite that enough is enough,' he said.

Liberals have complained about the term 'socialist' being used - such a label could hurt the Democratic nominee in the general election, making the person subject to attacks they will move the country to the left.

Donald Trump, in his campaign rallies, has accused Democrats of being led by its most liberal party members and suggested they want to turn the United States into a socialist country like Venezuela.

Now Biden is pushing the label as his campaign gets piled upon - a contrast to his early standing in the race when he was seen as the Democrat most likely to defeat the president in November 2020.

Warren has surpassed Biden in some polls as Democrats fret about the state of the Biden campaign - his fundraising, his poll numbers and now concerns he will flop in the all important Iowa caucuses.

It's less than 100 days before Democrats cast their first ballots to pick their 2020 nominee - the Iowa caucuses on February 3.

Biden's results in the 2008 Iowa contest ended his presidential bid that year when he came in with 1 per cent of the vote.

Now Democrats in the state are worried the former vice president will come in third or fourth when the caucuses take place next year.

Biden isn't spending time with the small groups of influential voters and party officials that he needs to do to garner support, Democrats in Iowa told Bloomberg.

In contrast they point to Warren and Pete Buttigieg for their well-organized efforts in Iowa.

Biden is trailing Warren by 5 points in the RealClearPolitics polling average for Iowa, with Buttigieg one point behind the former vice president.

Democrats in Iowa are fretting Joe Biden isn't spending enough time campaigning there - he takes a selfie with a voter in West Point, Iowa, last week

Democrats point to Elizabeth Warren and Pete Buttigieg as having highly-organized campaigns in Iowa

The Biden campaign told Bloomberg it's confident of its standing in the early-voting state, pointing out the former vice president has about 70 endorsements with many surrogates, including his wife Jill, campaigning in Iowa on his behalf.

'The baseline here is we feel really good about what we're doing,' said Pete Kavanaugh, Biden's deputy campaign manager. 'We're laser-focused on two things: Recruiting volunteers and precinct captains and talking to voters.'

A poor result in Iowa is unlikely to end the Biden campaign as the former vice president is expected to do well in South Carolina, an early voting state where he holds a strong lead in the polls.

But it's a sign of continued worry among Democrats about the strength of his operation, particularly after Warren, Sanders and Buttigieg outraised him in the third quarter of this year.

Democrats have been grumbling about all their primary choices as no contender has yet caught fire in the nomination contest.

The other candidates are sparking worry too, however - there are concerns that Warren is too liberal to win in the general election, about Sanders' health, and that Buttigieg may not be able to appeal beyond white voters.

It's a familiar position to Democrats to be searching for the unicorn - the perfect candidate that can unit the moderate and liberal wings of the party while appealing to the greater electorate in the general election, particularly those blue-collar voters in the Midwest who helped put Trump in the White House.

The party typically goes through a crisis of candidates during a presidential primary season and long for someone to rescue them from the slate before them.