Senator Elizabeth Warren’s formation of an exploratory committee for president on Monday represents a sort of political ball drop, the beginning of a new year and new campaign that will feature an unusually large number of Democrats seeking the presidency.

Or will it?

As the 2020 Democratic primary gets underway, the defining characteristic of this first stage of the race is the sheer uncertainty about who is even running. More than 30 Democrats are mulling presidential bids, but hardly any of them qualifies as an instant front-runner or a gifted, tested campaigner, and some of the biggest names could pass in the end.

Even beyond the field of specific candidates, this will be one of the most fluid nominating contests Democrats have had in recent decades. A leaderless party, they are in a moment of transition from centre-left politics to a more ideologically pure brand of liberalism.

Democrats are grappling with what they stand for, what their voters expect, and what demographic groups will be most critical in winning an Electoral College majority in 2020. But most of all, they are unified by a once-unimaginable objective that only illustrates how unpredictable American politics has become: ensuring Donald Trump is just a one-term president.

Who’s going to run?

It is hard to recall a recent presidential primary where, at the outset of the race, there was this much genuine mystery – not only about who would eventually emerge as the nominee, but who planned to run at all.

Ms Warren’s announcement on Monday was expected. And it will not be a surprise when senators Cory Booker of New Jersey and Kamala Harris of California enter the race in the coming weeks.

But the number of would-be candidates who may ultimately stay out of the race is larger than the list of contenders who are certain to run.

There are the well-known, or at least much-buzzed-about, Democrats who are still deliberating: former vice president Joe Biden is the most prominent of this group, and leads the field in initial polling in Iowa. But there is also great anticipation over Senator Bernie Sanders of Vermont, the runner-up for the Democratic nomination in 2016, and Texas representative Beto O’Rourke, whose losing senate bid this year electrified many grassroots Democrats.

Who could be running against Trump in 2020? Show all 23 1 /23 Who could be running against Trump in 2020? Who could be running against Trump in 2020? Joe Biden The former vice president - poised to be a frontrunner - has announced his run. He recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well AFP/Getty Who could be running against Trump in 2020? Bernie Sanders The 2016 runner-up has announced that he will be running again in 2020 Getty Who could be running against Trump in 2020? Hillary Clinton The 2016 Democratic presidential candidate and former Secretary of State says she is “still considering” whether she will run again. Getty Who could be running against Trump in 2020? Pete Buttigieg The Indiana mayor and war veteran will be running for president. If elected, he would be the first openly LGBT+ president in American history. Getty Who could be running against Trump in 2020? Kamala Harris The former California attorney general will be running for president in 2020. 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. AP Who could be running against Trump in 2020? Elizabeth Warren The Massachusetts Senator has formally launched her bid for president in 2020. A progressive Democrat, she is a major supporter of regulating Wall Street. AP Who could be running against Trump in 2020? Beto O’Rourke The former Texas congressman told Oprah Winfrey that he “has been thinking about running for presidency”, but stopped short of formally announcing his bid to run in 2020. AFP/Getty Who could be running against Trump in 2020? Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam has announced his bid. He intends 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 Who could be running against Trump in 2020? Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege.” Getty Who could be running against Trump in 2020? Cory Booker The New Jersey Senator has announced that he will be running for the presidency in 2020. If he secures the nomination he said finding a female vice president would be a priority. Getty Who could be running against Trump in 2020? John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017. AP Who could be running against Trump in 2020? 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 Who could be running against Trump in 2020? Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but is likely to face tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad. Getty Who could be running against Trump in 2020? Andrew Yang The entrepreneur has announced his presidential candidacy, and has pledged that he would introduce a universal basic income of $1,000 a month to every American over the age of 18. AFP/Getty Who could be running against Trump in 2020? Marianne Williamson The author and spiritual advisor has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful. Getty Who could be running against Trump in 2020? John Kerry The former secretary of state has said he is still thinking about whether to run. Getty Who could be running against Trump in 2020? Michael Bloomberg The entrepreneur and former New York mayor– with a net worth of around $50bn – has said he will decide by the end of February whether to seek the presidency. AFP Who could be running against Trump in 2020? Howard Schultz Former Starbucks CEO Howard Schultz has not yet ruled out running for president in 2020, despite criticism that his bid could help re-elect Mr Trump by dividing the Democrat vote. AP Who could be running against Trump in 2020? Eric Holder The former attorney general has said he will decide in “the next month or so” whether to run as a 2020 presidential candidate. AP Who could be running against Trump in 2020? Eric Swalwell The California congressman said he is “ready to do this” and will decide before April whether to run. MSNBC Who could be running against Trump in 2020? Terry McAuliffe The former Virginia governor, who worked to elect Democratic governors during 2018 midterms, said there was a “50 per cent” chance he would run. AP Who could be running against Trump in 2020? Sherrod Brown The Ohio senator is still undecided about whether to run for president in 2020. Who could be running against Trump in 2020? Mitch Landrieu The former New Orleans mayor said he doesn’t think he will run for president, but “never say never”. AP Photo/Jacquelyn Martin

It is unclear if any of these men will enter the race – particularly Mr Biden, who, associates say, is ambivalent about running after over three decades of presidential fits and starts.

There is also great uncertainty about lesser-known potential contenders who could be formidable – and if the Trump phenomenon proves anything, it is that making any assumptions about candidates is folly.

Will senators Sherrod Brown of Ohio and Amy Klobuchar of Minnesota – both of whom just won third terms in presidential battleground states – go beyond their flirtation with running? And what of the roster of current and former Democratic governors, like Steve Bullock of Montana or Terry McAuliffe of Virginia?

Meeting the moment

Presidential politics recalls the adage that 80 per cent of life is showing up.

Except in the case of primaries. Then, it’s about knowing when to show up.

That question, more than anything, is what looms over Ms Warren’s candidacy: would she be the president today if she had run in 2016, as some liberal activists and admirers urged her to? Mr Sanders ended up filling the void on the populist left and ran a surprisingly strong campaign against Hillary Clinton.

Ms Warren was hardly the first White House hopeful to risk waiting. Bill Clinton, for example, opted out of the 1988 campaign and still became president four years later.

But the more recent history of presidential calculations suggests that candidates are wiser to run when the moment presents itself. That is what Obama did in 2008 after just four years in the Senate, the same period Ms Warren would have served by 2016. Some Democrats think 2020 is Mr O’Rourke’s moment: he has been in the House for just six years, but many liberals see his energy and freshness as inspiring.

One reason Ms Warren announced on Monday and declared so early, her allies suggested, was to erase any uncertainty about her intentions among supporters whom she kept waiting – and ultimately disappointed – in 2016.

But now, instead of a head-to-head race against Hillary Clinton, Ms Warren is entering a wide-open campaign facing questions about her sensitivity on race and her political savvy after the release of a DNA test of her Native American heritage. Her decision to take the test angered some allies on the left and did nothing to stop Mr Trump’s mockery of her roots.

Democratic senator Mazie Hirono calls out Donald Trumps shutdown on live TV: 'Any efforts on his behalf to blame the Democrats would be such bullsh*t'

What do voters want?

Timing is also crucial in presidential races because the issues can change so quickly. Mr Trump’s incendiary rhetoric and policies around race and immigration have shifted the political conversation away from matters of economic inequality, which has been the life’s work of Ms Warren and which defined much of the 2016 Democratic primary.

Still, her party has plainly moved to the left on economics and social justice. So as much as anything, Ms Warren’s candidacy will test whether 2020 primary voters will demand that their candidates take unwavering positions on issues like universal health care and Wall Street regulations, much as they have historically expected them to do so on supporting abortion rights.

Mr Sanders’ 2016 candidacy demonstrated that there is hunger among Democrats to confront Wall Street excesses and income inequality. But what’s less clear is how much of a premium the voters will put on purity on these issues – and if they will overlook a softer line from a candidate who inspires them and who they think can defeat Mr Trump.

A warning shot at big money

Ms Warren is one of several potential candidates who have crusaded against the influence of wealthy donors and corporations, even to the point of calling out fellow Democrats for taking a more pliant approach to big-dollar fundraising and business regulation. And she is unlikely to be any less confrontational with rivals in the primary who are heavily backed by billionaire donors, or who are billionaires themselves.

That poses a threat to possible Democratic candidates like Michael Bloomberg and Tom Steyer, who are aligned with liberals on important issues like climate change and immigration but embody the kind of capitalist wealth that many despise. She may also cause trouble for non-billionaires like Mr Booker and Mr McAuliffe, who have collected enormous sums of money over the years from banks, hedge funds and pharmaceutical corporations. Mr Booker recently became the first candidate to have a super PAC formed in support of his anticipated campaign.

There is reason to expect Democratic primary voters and activists will be wary of candidates they see as too cosy with big money.

In the 2018 midterms, Democratic candidates who disavowed corporate money fared exceedingly well in primary elections, and the incoming House majority is expected to make ethics and campaign-finance legislation one of its first priorities. Revulsion from a money-soaked political system is one of a few concerns that links Democrats on the activist left, such as representative-elect Alexandria Ocasio-Cortez, with less ideological party favourites like Mr O’Rourke.

It is not difficult to imagine Ms Warren emerging as a kind of good-government buzz saw on debate stages dotted with billionaires and super PAC beneficiaries. Whether or not voters thrill to her persona or policies, there may be no candidate better equipped than she is to help set the rules of engagement around political money and the 2020 field – and to call out those who flout them.

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Splitting the screen with Washington

Ms Warren’s step into the 2020 race was one of the biggest political stories of the day, but even a Democrat of her stature had to share the news cycle with an ongoing government shutdown and Mr Trump’s demands that congress finance a border wall with Mexico. So it is likely to go for most of the 2020 campaign, as a president prone to political brinkmanship and angry outbursts simultaneously navigates divided government and his own battle for a second term.

Crises in government could routinely overshadow the Democratic primary campaign. Democrats’ internal debates about ideology and their party’s identity could be obscured by louder, more venomous clashes between the two parties. And challengers hoping to beat Mr Trump on the campaign trail may have difficulty competing for attention when he is in the White House – or on Twitter.

At times, there could also be moments of friction between the Democrats running for president and the Democrats helming the party in congress. In the case of the shutdown, there is relatively little daylight, at least so far: neither legislative leaders like Nancy Pelosi nor candidates like Ms Warren want to build a border wall. But in other fights – from cabinet nominations to possible impeachment proceedings – there may be a wider gap between would-be presidents striving to win primary votes and legislative tacticians aiming to outmanoeuvre Republicans or cut deals with them.

Candidates like Ms Warren, and other sitting senators like Mr Harris and Mr Booker, will each have to strike their own balance between joining the fights of the day in Washington – often in reaction to Mr Trump – and introducing themselves to voters around the country, outlining their agendas and drawing contrasts with their rivals for the nomination.