Ten Democrats seeking the White House will gather Thursday for their party's third debate of the 2020 cycle, with frontrunner Joe Biden sharing the stage with top rival Elizabeth Warren for the first time.

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 on stage 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, 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.