Former US vice-president Joe Biden will face Senator Elizabeth Warren for the first time in today's Democratic Party candidate debate in Houston, Texas, with polls showing the progressive Massachusetts Senator gaining on him.
While there are still 14 months left till the elections, the Democratic field has shrunk, and candidates are beginning to come into sharper focus.
The three-hour debate, set to start at 8am Singapore time, comes as Democrats are emboldened by President Donald Trump's dwindling appeal.
A Washington Post-ABC News poll published on Tuesday showed that his approval rating among voting-age Americans had fallen to 38 per cent from 44 per cent in early July. The poll also found Mr Biden leading Mr Trump by 55 per cent to 40 per cent.
There will be 10 candidates onstage at the debate, and of these, three are now seen as the strongest contenders for the party's nomination - Mr Biden, Vermont Senator Bernie Sanders and Mrs Warren. A possible fourth is California Senator Kamala Harris.
While the debate is not a "sudden death" - all will be back for a fourth round in mid-October - this one could be a bellwether.
Mr Biden's popularity is resilient, but he has been looking increasingly fallible, with questions persisting over his age - he is 76.
A new NPR/PBS NewsHour/ Marist poll released on Wednesday found that 75 per cent of registered Democrats and Democrat-leaning independents - a group that will be pivotal in deciding the 2020 race - now say they have a favourable impression of Mrs Warren, up from 53 per cent in January. The percentage of those with a negative impression of her has gone down from 17 to 11.
She leads Mr Biden, who polled 71 per cent. But among all registered voters, Mr Biden had the highest favourability of 45 per cent, followed by Mrs Warren at 41 per cent.
Mr Biden, who benefits - or not, from the Republican point of view - from his association with former president Barack Obama, is also popular among black voters, a critical but complex constituency.
Many black Evangelicals support Mr Trump. More broadly among those who support the Democratic Party, there is a generational split.
The former vice-president has the support of almost two-thirds of black voters aged 65 and older, according to a recent Morning Consult poll. And among all age groups, Mr Biden leads Mr Sanders by 41 per cent to 20 per cent.
"You go with what you know. A lot of black voters know Joe Biden," Mr Michael Nutter, a former Philadelphia mayor and a current Democratic National Committee member, told the journal Politico. "There's power in that and there's loyalty in that."
Unlike his closest rivals, Mr Biden is a centrist in a party that has swung left even as the Republican Party has swung to the right.
Mr Biden relates to individuals and audiences across a broad spectrum. He has suffered personal tragedy: the loss of his first wife and a daughter in a car accident, and a son to cancer. Coming from Scranton, Pennsylvania, he can empathise with blue-collar workers, though he left the state a long time ago.
The polls show he is not only the strongest contender to Mr Trump, but he may do better or as well with non-Democrats as with Democrats. In short, he has better crossover appeal than his opponents.
But in recent weeks, he has been mixing up facts, and making off-colour remarks - speculating, for instance, on what would have happened if Mr Obama had been assassinated, and telling a New Hampshire audience: "I want to be clear, I'm not going nuts."
He has drawn scorn from Mr Trump and raised eyebrows of analysts for downplaying the strategic threat from China that is the centrepiece of Mr Trump's foreign policy.
Others on stage at the debate will be South Bend, Indiana, mayor Pete Buttigieg; entrepreneur, lawyer, philanthropist and son of Taiwanese immigrants Andrew Yang; former Texas congressman Beto O'Rourke; Minnesota Senator Amy Klobuchar; and San Antonio, Texas, politician Julian Castro.
But it is Mr Biden, together with Mrs Warren and Mr Sanders, who will offer the clearest vision of the party's divisions and sense of unity.