Elizabeth Warren has stormed to the front of Democratic presidential candidates, topping a series of polls in crucial states that have effectively made her joint frontrunner with Joe Biden.

With much of the media and political punditry taken up by revelations about Donald Trump’s allegedly inappropriate phone call with the leader of Ukraine, and the decision by Nancy Pelosi to move forward with impeachment, the Massachusetts senator has quietly surged ahead.

What’s more, many observers believe the impeachment investigation could create more headaches for Mr Biden, who Mr Trump asked Ukraine’s Volodymyr Zelensky to investigate for alleged corruption.

There has never been evidence to support long-held accusations by conservatives that Mr Biden inappropriately used his position as vice president to push for the ousting of an allegedly corrupt Ukrainian prosecutor who was investigating an energy firm that employed his son, Hunter.

However, in the days since Democrats said they were proceeding with impeachment investigations involving six committees of the House of Representatives, Republicans have hit back with accusations about Mr Biden and his son that could harm his 2020 run.

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

“I refer to Warren and Biden as co-frontrunners. I don’t see how you could in fairness do anything but that. Of course the contest is dynamic and could change rapidly,” Larry Sabato, professor of politics at the University of Virginia, told The Independent.

“Biden ought to be very worried about Warren. His argument is basically his electability, and if he loses that, he is going to struggle. Warren’s problem is that loads of Democrats think she’s too far left to defeat Trump.”

Four polls used by the Democratic National Committee (DNC) to determine which candidates qualify for its debates this week gave Ms Warren a lead over Mr Biden in Iowa, the first state to hold a primary or caucus, and in New Hampshire, which is the second. A poll also placed her ahead of the former vice president in another early primary state, Nevada, where voters cast ballots on 22 February.

Meanwhile, a fourth poll carried out by Quinnipiac University gave Ms Warren a lead over Mr Biden nationally, with her on 27 points, him on 25 and Bernie Sanders in a distant third on 16 points. Another poll, carried out by Economist/YouGov, which is not officially counted by the DNC but is well respected, also gave Ms Warren a one-point advantage over Mr Biden nationally, scoring them 27 to 26.

The polling companies point out such a narrow gap is within the margin of error. Nevertheless, there has been a sense that as 76-year-old Mr Biden has been forced to answer questions about his age and mental acuity after a series of less than stellar debate performances, the 70-year-old Ms Warren has barely put a foot wrong.

Queer Eye's Jonathan Van Ness and Senator Elizabeth Warren speak on the phone

This week, she supported the move to proceed with an impeachment investigation of Mr Trump, but urged Democrats not to overreach on something so divisive.

“Right now, I’d like to just see us do the Ukraine issue because it is so clear and it is such a clear violation of law,” she told CNN. “The president is asking for help against one of his political rivals and asking a foreign government for a thing of value for himself personally. That’s against the law.”

She has consistently impressed voters with her grasp on policy issues, and showed in three successive debates she was able to display passion when she needed to. A number of pundits believe the Democratic race will turn into a choice between one of the two frontrunners – the centrist Mr Biden, or the progressive Ms Warren.

“It now appears likely to boil down to a two-way contest, one in which Democrats will have to decide whether to go big or go home,” analyst Charlie Cook, founder of the bipartisan Cook Political Report, wrote this week.

“In this case, going big is doing something bold, daring, and exciting but potentially risky – that is, going with Warren. Going home is to a more comfortable, familiar, but not terribly exciting place: Biden. Revolution versus restoration.”

In an indication of apparent concern among some of Mr Biden’s top advisors, The New York Times reported there had been discussions about whether the campaign should start taking money from large donors.

All the other major candidates have sworn off the idea of accepting money from a political action committee, or PAC, but some of Mr Biden’s supporters believe he may need corporate donations to fight off the accusations Republicans are already making about him. All candidates are conscious of the way Democrats have often failed to respond to Republican smears, as in the case of allegations made about Hillary Clinton in 2016, and John Kerry in 2004.

“The vice president is getting hit from all directions,” Mark Riddle, a political strategist who was involved in discussions, told the newspaper.

“A lot of us believe there should be a fair fight. He can hold his own with his campaign in these early states or whatnot, but there’s only so much incoming somebody can take without a response.”