Bernie Sanders leads all other candidates seeking the Democratic nomination for president and is the most likely candidate to beat Donald Trump, according to two polls that place the Vermont senator as the top contender as the Iowa caucus and early state primaries near.

In a CNN poll, 27 per cent of Democrat and Democrat-leaning independents support Mr Sanders as the Democratic nominee. Twenty-four percent of respondents support former Vice President Joe Biden. Both candidates polled double digits ahead of other forecasted front-runners Elizabeth Warren and Pete Buttigieg.

Though the CNN poll puts their neck-and-neck support within the margin of error, it's the first time that Mr Biden has fallen from a clear lead in a CNN poll.

The results arrive as Mr Sanders and Mr Biden spar over the former vice president's previous comments on social security, as video resurfaced of Mr Biden endorsing controversial spending cuts to the program.

In a campaign video, Mr Biden accused Mr Sanders of launching a "barrage of negative attacks" against the candidate while he refused to answer questions about his past support for those cuts.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

Results of the CNN poll show that Mr Sanders spiked seven percentage points over the last month following a similar December poll asking voters to pick their choice for the Democratic nominee.

The poll was conducted by SSRS from 16 January though 19 January among a random national sample of 1,156 people with an oversampling in battleground states.

Mr Sanders also claims the highest percentage of voters — 38 per cent — who said they would be "enthusiastic" about his nomination, rather than merely "satisfied" or "dissatisfied" or "upset".

Among the highest in the "satisfied" category, 42 per cent of respondents said they would be "satisfied but not enthusiastic" about Ms Warren's nomination, but only 29 per cent said they were "enthusiastic". Amy Klobuchar captured 40 per cent for "satisfied but not enthusiastic" and only 19 per cent for "enthusiastic".

Mr Sanders also is the candidate that most respondents said agrees with them on issues that matter most to them (at 30 per cent) as well as the candidate that understands the problems facing people like them (at 29 per cent).

He also has the largest support among non-white respondents (30 per cent) as well as white respondents (24 per cent).

The senator also leads against the incumbent president by the widest margin among the Democratic candidates when voters are asked to choose between two candidates in a hypothetical face-off in the general election, according to the results of a SurveyUSA poll.

In the SurveyUSA poll of 4,069 registered voters nationwide, Mr Sanders had a nine-point lead at 52 per cent to Mr Trump's 43 per cent.

Mr Biden was the runner-up, with the former vice president getting 50 per cent to Mr Trump's 43 per cent.

For a similar question in the CNN poll, only 24 per cent of respondents said Mr Sanders has the greatest chance of defeating Mr Trump in the general election. He came in second to Mr Biden — 45 per cent of respondents said he is the best chance of beating the incumbent.