For weeks, it was a peculiarity of the Democratic presidential nomination race for 2020 that one of the leading candidates in the pack hadn’t actually announced he was running. Now former vice-president Joe Biden is officially in the race. That’s great news for his supporters, but bad news for the party. His candidacy will exacerbate the rift between the Democrats’ moderate and progressive factions, potentially splitting the party and enabling the re-election of Donald Trump.

Trump is a vulnerable incumbent. He lost the popular vote in 2016 by nearly 3m votes and is only in the White House thanks to a narrow electoral college win. While he has continued to maintain strong support with his party’s white working-class base, he has alienated the large number of college-educated and suburban voters (especially women) who used to reliably vote Republican – but voted Democratic in the 2018 midterm elections.

If Trump continues to repel the suburban voters who reluctantly supported him in 2016 while failing to gain any new converts, his 2020 defeat seems statistically inevitable, even without taking into account any possible downturns in the economy. It’s an indication of Trump’s vulnerability that there are currently 22 Democrats running to replace him (according to the New York Times’ scoreboard), with another six considered likely or potential candidates.

Biden, with his high name recognition and legacy as Barack Obama’s two-term running mate, consistently leads the field of potential Democratic contenders. He is popular with Democratic voters nostalgic for Obama’s presidency, and he openly regrets not having run as his successor in 2016.

One of the biggest strikes against Biden is his age. If elected, he would be 82 by the end of his first term – and the oldest president in US history. Biden first won election to the Senate in 1972, at a time when Democratic presidential candidates such as Senator Cory Booker of New Jersey and congresswoman Tulsi Gabbard of Hawaii were toddlers. The mayor of South Bend, Indiana, Pete Buttigieg, who has rocketed to prominence over the past month, wasn’t even born until Biden had been in office for nearly 10 years. Many party leaders believe that a fresh, young candidate is needed to oppose the septuagenarian Trump, not a grizzled veteran of five decades of political combat.

But the larger argument against Biden comes from the progressive wing of the Democratic party, which sees him as a dinosaur holdover from the party’s unenlightened past. Already progressives have charged Biden with a litany of political errors (or crimes), including his maladroit actions as the chairman of the Senate judiciary committee during the 1991 Anita Hill-Clarence Thomas hearings (which allegedly showed his regressive attitudes toward women) and his support for Bill Clinton’s 1994 crime bill (which is interpreted as callousness about high levels of African American incarceration).

The recent accusations several women have made against Biden – that he hugged, nuzzled, sniffed and otherwise inappropriately interacted with them in ways that made them feel uncomfortable – are seen as further evidence that he’s out of step with a party that has been transformed by the #MeToo movement. Worse still, progressives fear, Republicans will capitalize on the image of “Handsy Joe” to deprive Democrats of the moral high ground on sexism against the self-proclaimed “pussy-grabber” in the White House.

More generally, a Biden candidacy doesn’t fit with what is becoming the progressive wing’s theory of how to approach the 2020 election. According to this theory, the Democrats should nominate an exciting, charismatic candidate who will run on a bold, uncompromising agenda of radical change. It’s undeniable that over the past two years, the energy and momentum among party activists has shifted to the left. They’re tired of settling for half-measures; they want the whole chalupa.

Biden, with his name recognition and legacy as Obama’s two-term running mate, consistently leads the field of potential Democratic contenders Photograph: Jacquelyn Martin/AP

And it’s true that after eight years of Obama’s cautious incrementalism and two years of Trump’s chaos, there is a real opening for policies that would make capitalism function more equitably for the middle and working classes. This has been demonstrated recently by widespread popular support for the proposals of the congresswoman Alexandria Ocasio-Cortez (the self-proclaimed socialist and Democratic firebrand from New York) to raise taxes on wealthy Americans to 70%.

One variant of the progressive theory would call for the nomination of the socialist senator Bernie Sanders of Vermont, who came close to wresting the 2016 Democratic nomination away from Hillary Clinton by campaigning almost solely against income inequality and the many ways that the economy is tilted in favor of the very rich. Sanders is currently second in most polls of potential Democratic candidates, behind Biden. However, progressive activists’ concerns about identity and discrimination have risen to the fore very quickly and prominently in the last few years, leaving Sanders – who is, after all, a white man who’s even older than Joe Biden – struggling to keep up.

Increasingly, progressives call for the nomination of a candidate who represents a real break with the past – preferably a woman (like Senator Elizabeth Warren of Massachusetts), a minority (like Booker), or both (such as Senator Kamala Harris of California). And although those candidates already are the three most left-leaning members of the US Senate, progressives want them to go even further on both economic and cultural fronts. They believe such an uncompromising, unapologetically leftwing candidacy would excite what they believe is the Democratic base – young people, minorities and women – thus boosting turnout and guaranteeing victory.

Progressives point to the example of Stacey Abrams, who made history in 2018 as the country’s first major-party African American female gubernatorial nominee. She ran on an array of progressive causes (insisting that “Democrats can’t win by pretending to be Republicans”) and united a rainbow coalition of diverse supporters. She lost the election but, in the conservative-dominated state of Georgia, came closer to winning than any other Democratic candidate had in two decades. Many progressives, in fact, pine for Abrams to be the party’s 2020 presidential nominee.

A Biden presidential candidacy, in this view, would be a repeat of Hillary Clinton, who lost in 2016 because not enough Democrats were excited by her status quo candidacy. Progressives maintain that it’s not just age that disqualifies Biden from running as a champion of bold new change. He is too chummy with Republicans from his long political service – just recently he had to apologize to LGBTQ advocates for referring to the vice-president, Mike Pence, (long perceived as homophobic) as “a decent guy”. Although Biden is conservative only in relative terms, he is openly skeptical of some items on the progressive wish list – for example, he is one of the few potential or declared Democratic presidential hopefuls who has not yet endorsed the Green New Deal advanced by Ocasio-Cortez. And, needless to say, he is a straight white man, which makes him an existential obstacle (if not a threat) to many progressives.

Democratic voters will have to decide whether to endorse the progressive or realist strategies in 2020; they can’t choose both

The progressive theory of how to win in 2020 is countered by the view of what might be termed Democratic realists. The realists argue that America is a closely divided nation and that most Americans have limited tolerance for extremism of either the right or left. The lesson of the 2018 elections, according to this theory, is that Democrats won back the House because they ran mostly pragmatic, non-ideological candidates in the suburban districts where college-educated voters had experienced buyers’ remorse toward Trump. These voters might give a sympathetic hearing to bold proposals to raise taxes on billionaires or lower housing costs, but they’re going to be turned off by extremes of identity politics or socialist economics.

The only way that Trump can win reelection, according to the realists, is if the Democrats sabotage their chances by nominating someone too far outside the mainstream. Joe Biden probably would offer Democrats the best chance to win, in this view. So too could someone like the Senator Amy Klobuchar of Minnesota, or the former Colorado governor John Hickenlooper. But others could be acceptable, perhaps even including progressive favorites like Harris and Warren – so long as they refrain from the temptation to win the nomination by outflanking their rivals on the left, along the lines desired by progressives.

More strategically, the realists see the nomination of Biden or some other comparative moderate as necessary to head off the threat of a centrist, third-party candidacy. In 2016, the billionaire Mike Bloomberg would have run as an independent if the Democratic presidential nomination had gone to Sanders. In 2020, Bloomberg or someone like his fellow billionaire, the Starbucks CEO Howard Schultz, may feel that they have an opening if centrist voters face a choice between Trump and a far-left Democrat.

That could be bad news for Democrats. The Wason Center pollster Rachel Bitecofer, whose advance predictions of a Democratic blue wave in the 2018 House elections proved remarkably accurate, currently estimates that a generic Democrat would beat Trump by 11 points in a head-to-head race in 2020. But she also estimates that an independent candidate would draw off five Democratic voters for every Republican voter, meaning that Trump would win with a plurality.

A Trump re-election would be the ultimate nightmare for both Democratic progressives and realists. But it could happen if the split between the two factions becomes too deep and bitter, or if an extreme-left Democratic candidate provokes an independent to enter the race. Democratic primary voters will have to decide whether to endorse the progressive or realist strategies in 2020; they can’t choose both.