Hillary Clinton's once-robust lead over her presidential rival, Sen. Bernie Sanders, has evaporated completely, according to a poll released Friday.

A national Fox News Channel poll shows the Vermont democratic socialist Sanders with support from 47 per cent of Democratic primary voters. Clinton stands three points behind with 44 per cent.

Their positions were reversed when the same pollsters surveyed Democrats a month ago, with the former secretary of state leading him by a whopping 12 points, with a 49-37 margin.

The 15-point swing reflects Clinton's relative weak razor-thin victory in Iowa and Sanders' 22-point drubbing of her in New Hampshire.

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Feel it: Now Hillary Clinton is behind Bernie Sanders and has the prospect of a race to the wire in Nevada and Super Tuesday with her rival having all the momentum

Last summer she led Sanders by as many as 46 points. But with attention focused on her classified email scandal and renewed interest in her State Department's failures surrounding the 2012 terror attack on a diplomatic compound in Benghazi, Libya, all bets are off.

A 55 per cent majority of American voters now say Clinton lacks the 'integrity' to be an effective president. That includes 64 percent of independents.

And 60 percent of voters say Hillary's mishandling of classified emails put America's national security at risk.

That inclues 33 percent of Democrats and 59 percent of independents.

Clinton's much-anticipated 'southern firewall,' a presumed safety net built from minority voters in the deeep south, will be tested a week from Saturday when Democrats in South Carolina go to the polls.

With Sanders now ahead of her nationally, she may no longer be able to count on enough support to win her party's presidential nomination.

'One thing that is clear from our poll – and others – is that Clinton has been losing support and Sanders has been gaining,' Democratic pollster Chris Anderson said in a statement released by Fox News.

'And this process appears to have accelerated since the contests in Iowa and New Hampshire.'

Republican pollster Daron Shaw said that '[h]istorically, lesser-known candidates beating establishment candidates in early contests have seen the biggest boost in their national support.'

Drilling down into the poll's numbers reveals a shocking trend that has Clinton losing support among women.

Her unique selling point is that she would be the first female U.S. president, but other women are increasingly unimpressed.

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Her lead among female Democrats nationally is just 3 percentage points now, compared with 28 points a few months ago.

But she trails Sanders in another important category – electability against Republican front-runner Donald Trump.

Sanders would beat The Donald by a 53-38 margin if the election were held today according to the Fox polls. Hillary's margin of victory over the billionaire would be far smaller, just 5 points, at 47-42.

Bernie is also endearing himself to skeptical voters, with 30 per cent saying they would be either 'extremely' or 'very' satisfied if he were to becoe president.

That may seem like a small numbers, but Clinton scored just 24 per cent on that question. Trump polled 21 per cent.

On the other hand, fewer voters would be 'not at all' satisfied if Bernie were to win the White House – just 37 per cent – compared with Clinton's and Trump's metrics on that question, 49 and 55 per cent, respectively.