Criticism came after he said she got 'schlonged' by Obama back in 2012

She said he had a 'penchant for sexism' and ran a bulling campaign

Hillary is also taking a more combative stance on Trump and the election

Until now Bill had kept a relatively low profile in his wife's campaign

Hillary Clinton is preparing to unleash her 'secret weapon' in the bid for the presidential election campaign - her husband Bill.

The former secretary of state is sending her husband out to key early states such as Iowa and New Hampshire where she is locked in a 'dog fight' with Bernie Sanders for the Democratic bid.

Bill Clinton, who was ranked as the most popular president of the past 25 years in a recent survey, will be a 'huge asset' to Hillary who must secure one or both states. Losses in both would have a deep impact on her 2016 election bid.

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Hillary Clinton is preparing to release her 'secret weapon' in the bid for the presidential election campaign - her husband Bill (pictured together in July)

Marc Lasry, a friend of Clinton's and head of New York hedge fund firm Avenue Capital Group, told the Wall Street Journal: 'People love seeing (Bill) and he's able to explain things to people in a way that's unique.'

Hillary may need every bit of help she can after a recent CNN poll showed that the former first lady is now in a statistical tie with Republican front-runner Donald Trump.

The same stats also appear to show that her lead in the Democratic presidential primary is slowly evaporating.

Clinton once enjoyed a brief flash of world-beating dominance over Bernie Sanders in the primary fight, but that moment has passed as the margin she once enjoyed over The Donald has also shrunk.

The CNN poll shows that her lead over the Vermont socialist senator is back to just 16 percentage points – a 50-34 spread – down from 28 just three weeks ago.

The former secretary of state is sending her husband out to key early states such as Iowa and New Hampshire

And in a slugfest with Trump, she is now just 2 points ahead in a poll whose margin of error is 3.

Even if Clinton is a shoe-in for her party's nomination next year – an uncertain prospect given the specter of an FBI investigation into classified emails hanging over her head – Democrats are generally less enthusiastic than Republicans about voting in 2016.

CNN's polling data showed that 36 per cent of Republicans and GOP-leaning independents say they are 'extremely' enthusiastic about going to the polls next November, just 19 per cent of Americans on the Democratic side of the ledger agree.

Perhaps that explains why Clinton is so keen to use her husband - who she has branded her 'secret weapon' to bolster her appeal.

Until now, former president Bill Clinton has been keeping a relatively low profile, only appearing at a few private fundraisers and offering advice to Hillary behind closed doors.

But from next month, he will be taking a starring role at public forums for Hillary in Iowa and New Hampshire.

However, some have warned Hillary not to allow Bill to overshadow her own bid for the White House.

Clinton is said to be locked in a close battle with fellow candidate Bernie Sanders for key states Iowa and New Hampshire

NUMBERS: Hillary Clinton's lead over Bernie Sanders shrunk (left) between late November (green) and this past week (purple), but she clearly got a debate 'bounce' from Saturday (right)

'He's a luminescent figure. That's always an issue,' said David Axelrod, a senior adviser in both of Mr. Obama's presidential campaigns, who recommended that the couple campaign separately.

'It's important for people to see her out there on her own,' he added to the Wall Street Journal.

Mr Clinton remains a hugely popular figure in the Democrat party and was a surrogate for President Barack Obama during his re-election bid in 2012.

With his help, and following a strong debate performance Saturday in Manchester, New Hampshire, Mrs Clinton will be in a good position to pull ahead from Sanders in the key state.

While Clinton still leads against Sanders, Hillary and The Donald are splitting the support of independent voters, a crucial swing bloc that politicians especially covet.

The Democratic front runner appears to be taking a more combative stance to tackle Trump after the billionaire mogul said last week that President Obama had 'schlonged' her in the 2008 race to the White House.

Hillary, he said Monday in what might be the most brutal salvo in the 2016 general election, 'got schlonged' by the current occupant of the Oval Office when Obama came out of nowhere to snatch the presidential nomination from her.

HERE COMES DONALD: Trump's resilience and massive campaign crowds have put him in a statistical tie with Hillary Clinton when pollsters ask about a hypothetical general election battle between the two

'Hillary – that's not a president ... everything that's been involved in Hillary has been losses,' he said. 'You take a look.

'She was going to beat Obama! I don’t know who’d be worse – I don’t know, how does it get worse?' he asked.

'But she was gonna beat – she was favored to win and she got schlonged. She lost. I mean, she lost.'

He also made comments in the press about Clinton's brief absence from a recent Democratic debate stage, when she was reportedly using the restroom, which he called 'disgusting.'

Clinton later said she 'deplored the tone of his campaign' which she described as 'bullying'.

'The inflammatory rhetoric that he is using to divide people,' Clinton said, 'and his going after groups of people with hateful, incendiary rhetoric.'

'I don’t know that he has any boundaries at all. His bigotry, his bluster, his bullying have become his campaign. And he has to keep sort of upping the stakes and going even further.'

In an interview with the Des Moines Register, Mrs. Clinton added that Trump has 'demonstrated a penchant for sexism.'

Clinton's chief PR strategist, Jennifer Palmieri, tweeted: 'We are not responding to Trump, but everyone who understands the humiliation this degrading language inflicts on all women should.'