Bernie Sanders and Elizabeth Warren's feud has erupted into open warfare as it was reported he told her a woman can't win the presidential election.

The two senators met at Warren's Washington D.C. apartment in December 2018 to discuss their presidential runs and agreed to remain civil to one another on the campaign trail.

Warren then told him she thought should could win based on her economic policy and her appeal to female voters, to which Sanders responded a woman couldn't win the presidential election, CNN reported.

The network cited four sources familiar with the meeting in its report.

Sanders, however, denied he made such a remark.

'It is ludicrous to believe that at the same meeting where Elizabeth Warren told me she was going to run for president, I would tell her that a woman couldn't win,' Sanders told CNN in a statement. 'It's sad that, three weeks before the Iowa caucus and a year after that private conversation, staff who weren't in the room are lying about what happened. What I did say that night was that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could. Do I believe a woman can win in 2020? Of course! After all, Hillary Clinton beat Donald Trump by 3 million votes in 2016.'

Warren's campaign didn't comment and the two candidates could be asked about it in Tuesday night's presidential debate.

The two progressive candidates - who describe themselves as friends - have turned to attack one another as the battle for the Iowa caucus fight heats up ahead of Tuesday's crucial debate.

The bickering has opened a new window into their political rivalry, which they have carefully managed to keep in the shadows through public displays of unity, such as having their arms around each other at Democratic debates.

But there are also fears among Democratic strategist that a woman cannot beat President Donald Trump, pointing to Hillary Clinton's failure to do so in 2016.

Bernie Sanders has gone into attack mode after an influential poll showed him in lead in Iowa

But Sanders denied a CNN report he told Elizabeth Warren a woman can't win the presidential election

The bickering between Bernie Sanders and Elizabeth Warren has opened a new window into their political rivalry, which they have carefully managed to keep in the shadows through public displays of unity such as their hug at the July presidential debate

Sanders' campaign has gotten more aggressive in recent day and has let loose on Warren, Joe Biden and Pete Buttigieg - his leading competition for the state - after an important poll showed him leading the battle for the Iowa caucuses.

It's a new aggressive stance for the Vermont senator and the fierce criticism could preview a fiery debate for voters on Tuesday as the first contest for the Democratic presidential nomination approaches on February 3.

But it's Sanders' dust up with Warren, with whom he is friends and shares a progressive agenda, that is garnering the most attention.

His campaign has a script for volunteers that describes the Massachusetts senator as a candidate of the elite who can bring no new voters to the Democratic Party.

'People who support her are highly-educated, more affluent people who are going to show up and vote Democratic no matter what,' the script, obtained by Politico, says and adds: 'She's bringing no new bases into the Democratic Party.'

Warren said she was disappointed in Sanders' actions. The two have been long time supporters of one another and share a similar agenda, including Medicaid For All and free college tuition.

'Bernie knows me, and he's known me for a long time,' Warren told reporters this weekend while she campaigned in Iowa

'I was disappointed to hear that Bernie is sending his volunteers out to trash me,' she said. 'I hope Bernie reconsiders and turns his campaign in a different direction.'

She warned Democrats don't want a repeat of 2016 when Sanders' supporters and senator himself didn't embrace nominee Hillary Clinton after a bitter party primary.

'We all saw the impact of the factionalism in 2016, and we can't have a repeat of that,' Warren said. 'Democrats need to unite our party and that means pulling in all parts of the Democratic coalition.'

Democratic presidential contenders have flooded Iowa this month ahead of Tuesday night's debate and the February 3 caucuses.

Only six contenders will be on the debate stage - Biden, Sanders, Warren, Buttigieg, Amy Klobuchar and Tom Steyer.

As the caucuses approach, the in-fighting could get worse.

Sanders, who garnered the top spot in the influential Des Moines Register/CNN/Mediacom Iowa Poll on Friday for the first time, shrugged off the report of friction with Warren, calling it a 'media blow up' and saying he and Warren are 'friends.

Sanders also attacked Joe Biden, saying Biden 'doesn't really have any volunteers' and that 'no one is really excited about him.'

Sanders has also attacked Pete Buttigieg, saying he doesn't have support among African Americans

'I got to tell you, I think this is a little bit of a media blowup, that kind of wants conflict,' the Vermont senator told reporters in Iowa.

'Elizabeth Warren is a very good friend of mine,' he noted. 'We have worked together in the Senate for years. Elizabeth Warren and I will continue to work together, we will debate the issues.'

'No one is going to trash Elizabeth Warren,' Sanders added. 'We have hundreds of employees. Elizabeth Warren has hundreds of employees. And people sometimes say things that they shouldn't. You have heard me give many speeches. Have I ever said one negative word about Elizabeth Warren?'

The script - to be used by volunteers in Iowa calling voters or knocking on doors - also attacks Sanders' fellow front runners Biden and Buttigieg.

It notes Buttigieg lacks support among African Americans and young people while it says Biden 'doesn't really have any volunteers' and that 'no one is really excited about him.'

Sanders also attacked Biden during a Democratic forum in Iowa Sunday, reminding voters he opposed the 2002 authorization for military force in Iraq, which Biden voted for.

'In 2002, I helped lead the effort against the war in Iraq, which turned out to be the worst foreign policy blunder in the modern history of America,' Sanders said. 'The war in Iraq was based on a series of lies.'

The Sanders campaign also attacked Biden on his vote for the Iraq War, issuing a statement calling it the 'worst foreign policy blunder in modern American history.'

Amy Klobuchar will be among the six contenders on the debate stage in Des Moines Tuesday night

Attendees listen as Pete Buttigieg speaks during a campaign event on Sunday

'It is appalling that after 18 years Joe Biden still refuses to admit he was dead wrong on the Iraq War, the worst foreign policy blunder in modern American history,' Sanders senior advisor Jeff Weaver said in a statement Sunday. 'Bernie Sanders saw the same information and had the judgment to vote against the Iraq War. That's the kind of commander in chief we need — someone with the toughness and judgment to get those calls right, not someone who undermined Democratic opposition, enthusiastically supported a disastrous war, refuses to admit mistakes, and then tries to rewrite history.'

It's a notably aggressive stance from Sanders, who usually focuses on policy positions and not attack lines in his debate performances and campaign stops.

The senator said he is simply running a presidential campaign.

'We are going to be talking about the record,' he said. 'People are talking about my record. I was just asked a question about my record. That's kind of what a campaign is about. We will contrast records — nothing wrong with that.'

Even President Donald Trump has noticed Sanders' rise in the polls.

'Wow! Crazy Bernie Sanders is surging in the polls, looking very good against his opponents in the Do Nothing Party. So what does this all mean? Stay tuned!,' he tweeted on Sunday.

The winner of the Iowa caucuses has gone on to be the Democratic nominee in nine of the 11 cycles since 1976.

The Democratic field in Iowa is in flux as Friday's poll showed. Buttigieg, who had been leading, dropped to third place behind Sanders' lead and Warren's second spot. Biden came in fourth.

But Sanders' three point lead in the poll is narrow and within the margin of error - an indication the race for the state is wide open.

Additionally the senator has lead fundraising among the contenders, giving him the funds to stay in the contest for the long hall, leading to concerns among Democratic strategists that a bruised and exhausted nominee will emerge to take on President Trump in November.