Joe Biden maintains a grip on pole position with 26.8% support, despite a noticeable dip in recent weeks.

Frontrunner Joe Biden goes head-to-head for the first time Thursday night with Elizabeth Warren, his fast-rising rival for the Democratic Party's presidential nomination, in a three-hour debate seen as a tough test of the 76-year-old's stamina.

Ten candidates will take the stage in Houston as a narrowed 2020 field, including Bernie Sanders, the liberal senator jockeying for second spot with Warren in the race to challenge President Donald Trump in 2020.

While all three Democrats are in their seventies, all eyes will be on former vice president Biden who is battling accusations that he a gaffe-prone candidate past his prime, who could struggle during a gruelling campaign.

The previous debates, in June and July, featured a dizzying 20 hopefuls over two long nights, with seven months to go before the first votes are cast in Iowa to pick the nominee.

While most of them remain in contention, the lineup has been tightened this time around for the televised showdown, airing on ABC from 7:00 pm (2300 GMT).

Bringing 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; 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 two most prominent progressives, Warren and Sanders.

Biden maintains a grip on pole position with 26.8 percent support, despite a noticeable dip in recent weeks, according to a poll average compiled by RealClearPolitics.

A summer of verbal miscues -- and an apparent lack of preparedness for spirited attacks by rivals in the first debate -- raised doubts about his age and mental clarity.

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.

"Democrats hoping for Biden to collapse may find themselves in the position of Donald Trump's opponents from four years ago," when the now-president swatted away a historically large field of rivals, said Larry Sabato, who heads the Center for Politics at the University of Virginia.

Fireworks?

There is potential for fireworks between Biden and Warren, who trails at 16.8 percent but is the one candidate who has enjoyed a steady rise in support.

Warren, 70, has electrified town halls and impressed voters with her extensive collection of policy platforms.

Sanders, at 78 the oldest candidate in the field, is narrowly leading Warren at 17.3 percent support, and 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 (6.5 percent) and 37-year-old South Bend Mayor Pete Buttigieg (4.8 percent) have stalled.

For the second tier, including ex-congressman Beto O'Rourke (2.8 percent), Senator Cory Booker (2.3 percent), Senator Amy Klobuchar (1.2 percent) and Obama-era housing secretary Julian Castro (1.0percent), a breakout moment is critical to get noticed in a field of heavyweights.

O'Rourke and Castro, both from Texas, enjoy home-field advantage for the debate. Hours ahead of the event, O'Rourke placards covered telephone poles outside the venue.

Navigating between the leaders and the strugglers is tech entrepreneur Andrew Yang (3.0 percent), the only non-politician on the stage and the surprise performer of the campaign, whose universal basic income plan has drawn attention.

Beyond the economy, 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.