The televised showdown in Houston will be the longest to date in the primary contest, a three-hour marathon that will give voters their first opportunity to see all the leading candidates onstage together.

The previous encounters, in June and July, featured 20 candidates over two nights, leaving viewers shell-shocked a full seven months before the first votes are cast, in Iowa next February, to determine the nominee.

While most of those candidates remain in the race, the debate is effectively halving the field as Democrats seek a challenger to President Donald Trump.

Thursday's debate, airing on ABC from 7:00 pm (2300 GMT), will bring America's diversity to the fore.

The 10 candidates are white, black, Hispanic, and Asian-American; seven men and three women; three septuagenarians and four candidates 30 to 40 years their junior; and centrists, progressives and liberals.

In a sign of the different political currents coursing through the party, the moderate Biden will take center stage sandwiched between the prominent progressives Warren and Senator Bernie Sanders, a self-described Democratic socialist who launched the universal health care approach known as Medicare for All.

All eyes will be on 76-year-old Biden, who maintains a grip on pole position with 29.8 percent support, according to a poll average compiled by RealClearPolitics.

His summer of verbal miscues -- and an apparent lack of preparedness for spirited attacks by rivals in the first debate -- raised doubts about Barack Obama's deputy's age and mental clarity, and whether he will stand the test of a grueling political campaign.

To date, the Democratic veteran has largely kept those concerns at bay.

He enjoys strong support in particular from African-American communities and from working-class whites who appreciate his blue-collar appeal and believe he is best able to beat Trump, a top priority for Democratic voters.

'Messier' campaign ahead? 

There is potential for fireworks between Biden and Warren, who trails at 18 percent but is the one candidate who has enjoyed a steady rise in support.

Warren, 70, has electrified town halls and other campaign events with her extensive collection of policy platforms.

"If Warren has another strong debate and continues gaining in the polls, for instance, we might have a relatively clear two-way race between her and Biden," wrote statistician and elections specialist Nate Silver on the website FiveThirtyEight.com.

"But the reality will probably be a lot messier."

Sanders, at 78 the oldest candidate in the field, narrowly trails Warren at 17.7 percent support, and he has largely avoided clashing with his Senate friend and fellow progressive.

While Warren's stock has risen, the campaigns of others such as Senator Kamala Harris (seven percent) and 37-year-old South Bend Mayor Pete Buttigieg (4.3 percent) have stalled.

For the second tier, including Senator Cory Booker (2.5 percent), ex-congressman Beto O'Rourke (2.3 percent), Senator Amy Klobuchar (1.0 percent) and Obama-era housing secretary Julian Castro (0.8 percent), a breakout moment is critical to get noticed in a field of heavyweights.

Navigating between the leaders and the strugglers is tech entrepreneur Andrew Yang (2.7 percent), the only non-politician on the stage and the surprise performer of the campaign, whose universal basic income plan has drawn attention.

Three topics -- gun control, climate change and health care -- are shaping up as defining issues for Democrats, at least in the early stages of the campaign, and they will likely be addressed at length at the debate.

Lawmakers returned to work this week after a summer recess marred by several mass shootings, including one in O'Rourke's home town of El Paso that left dozens of people dead, and most of the Democratic presidential hopefuls have demanded Congress take steps to address the violence.

Several candidates who failed to make the cut for Thursday's debate have vowed to stay in the race, including billionaire activist Tom Steyer and New York Mayor Bill de Blasio.

But Silver, the statistician, offered a bleak prediction for the lower-ranked candidates: "Unless they turn it around soon, they're probably toast."

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