(Adds comment from Biden advisers)
By Tim Reid and Joseph Ax
HOUSTON, Sept 12 (Reuters) - Leading Democratic White House contenders Joe Biden and Elizabeth Warren will share the stage for the first time on Thursday when the top 10 candidates for the party's presidential nomination meet in a debate in Houston.
The third debate in the race to find a challenger to Republican President Donald Trump in the November 2020 election will spotlight the party's ideological divisions, with moderate front-runner Biden at center stage flanked by progressives Warren of Massachusetts and Bernie Sanders of Vermont, both U.S. senators.
Seven other Democrats, including U.S. Senators Kamala Harris of California and Cory Booker of New Jersey, and South Bend, Indiana, Mayor Pete Buttigieg, also will participate as all of the party's highest-polling candidates square off for the first time.
Earlier debates in June and July were split over two nights to accommodate a crowded field of 20 contenders. But the party toughened the qualifying criteria to winnow the field and give a bigger spotlight to the top candidates.
"For us, it's a great opportunity to advance," Buttigieg told reporters in New Hampshire last weekend. "It's still 10 at a time, but now we at least know looking at that stage that the nominee will emerge from that group."
The first two rounds of debates have been contentious. Biden came under repeated attacks led by Harris and Booker for his record on race and criminal justice reform and his views on healthcare.
Warren and Sanders, old allies who have promised not to criticize each other, teamed up during the second round of debates in Detroit to defend their progressive agenda when attacked by more moderate candidates.
But this time, Warren could be a bigger target for criticism - and have more incentive to criticize Biden. A steady rise in opinion polls over the past two months has moved her ahead of Sanders into second place nationally in the Real Clear Politics polling average.
Warren and Biden have tangled before. She challenged him more than a decade ago when she was a law professor over his support for a bill to make it harder to declare bankruptcy for personal debts. Biden represented Delaware, home to several large credit card companies, in the U.S. Senate.
Senior Biden campaign officials told reporters the former vice president would emphasize that he was the candidate with the longest record of accomplishment, and the true defender of former President Barack Obama's legacy.
While Warren has rolled out a series of policy proposals, Biden will stress that candidates need to show they can actually make real progress, not just plans, the advisers said.
Biden has held his front-runner status in opinion polls since he entered the Democratic nominating race in April, with Warren and Sanders battling for second behind him. None of the other contenders reaches double digits in most polls at this point.
'GO ON THE ATTACK'
For the other contenders at the edges of the stage, the task will be simply to make an impression.
"You may have candidates on the stage who have no other option but to go on the attack," said Chris Kofinis, a Democratic strategist who worked on the presidential campaign of former U.S. Senator John Edwards.
"If you are any of the bottom seven, you have to say something or do something to stand out. It's simple math," he said.
The sharp bickering during the first two rounds of the debates dismayed some Democrats, who have urged the candidates to rein in their attacks and focus on laying out their own affirmative agendas.
"A lot of us have talked to the candidates directly, and I think you are going to see people really offering a vision," said Ray Buckley, chairman of the New Hampshire state party. "Right now, this country does not need to hear about Democrats squabbling."
Also participating in the debate in Houston will be U.S. Senator Amy Klobuchar of Minnesota, former U.S. Housing Secretary Julian Castro former U.S. Representative Beto O'Rourke and entrepreneur Andrew Yang of New York.
Castro, also the former mayor of San Antonio, and O'Rourke are both from Texas, which will vote on March 3 - Super Tuesday - early in the nominating schedule next year.
The Houston debate, to be televised by ABC News, is scheduled to last for up to three hours.
"This is a great opportunity for me. We'll have more people watching because it's just one night," Klobuchar told reporters.
Many of the candidates left off the stage have grumbled about the Democratic National Committee doubling the fundraising and polling requirements from the first two debates.
One more contender, billionaire activist Tom Steyer, managed to qualify last week for the October debate. That ensures two nights of Democratic debates again next month.
(Reporting by Tim Reid and Joseph Ax; Additional reporting and writing by John Whitesides; Editing by Peter Cooney and Jonathan Oatis)
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