Biden's remarkably steady lead in the crowded contest has been built on the idea that the former vice president is best suited to defeat President Donald Trump next year — a contention based on ideology, experience and perhaps gender. Sanders and Warren, meanwhile, have repeatedly criticized Biden's measured approach, at least indirectly, by arguing that only bold action on key issues like health care, the economy and climate change can build the coalition needed to win in 2020.
The top-tier meeting at center stage has dominated the pre-event talk, yet each of the other seven candidates hopes for a breakout moment with the attention of the nation beginning to increase less than five months before the first primary votes are cast.
"For a complete junkie or someone in the business, you already have an impression of everyone," said Howard Dean, who ran for president in 2004 and later chaired the Democratic National Committee. "But now you are going to see increasing scrutiny with other people coming in to take a closer look."
The ABC News debate is the first limited to one night after several candidates dropped out and others failed to meet new qualification standards. A handful more candidates qualified for next month's debate, which will again be divided over two nights.
Beyond Biden and Sens. Warren and Sanders, the candidates on stage Thursday night include Minnesota Sen. Amy Klobuchar, New Jersey Sen. Cory Booker, South Bend, Indiana, Mayor Pete Buttigieg, California Sen. Kamala Harris, New York businessman Andrew Yang, former Texas Rep. Beto O'Rourke and former Obama administration Housing chief Julian Castro.
Viewers will see the diversity of the modern Democratic Party. The debate, held on the campus of historically black Texas Southern University, includes women, people of color and a gay man, a striking contrast to the Republicans. It will unfold in a rapidly changing state that Democrats hope to eventually bring into their column.
Perhaps the biggest question is how directly the candidates will go after one another. Some fights that were predicted in previous debates failed to materialize with candidates like Sanders and Warren in July joining forces.
The White House hopefuls and their campaigns are sending mixed messages about how eager they are to make frontal attacks on anyone other than Trump. That could mean the first meeting between Warren, the rising progressive calling for "big, structural change," and Biden, the more cautious but still ambitious establishmentarian, doesn't define the night. Or that Harris, the California senator, and Buttigieg, the mayor of South Bend, Indiana, look to reclaim lost momentum not by punching rivals but by reemphasizing their own visions for America.
Trump himself, in Baltimore to address GOP lawmakers at a retreat, said of the debate, "I'm going to have to watch it as a re-run." He predicted the Democrats' nominee would ultimately be Biden, Warren or Sanders.
Biden, who has led most national and early state polls since he joined the field in April, downplaying the prospects of a clash with Warren, despite their policy differences on health care, taxes and financial regulation.
"I'm just going to be me, and she'll be her, and let people make their judgments. I have great respect for her," Biden said recently as he campaigned in South Carolina. Warren says consistently that she has no interest in going after Democratic opponents. Yet both campaigns are also clear that they don't consider it a personal attack to draw sharp policy contrasts.
Warren, who as a Harvard law professor once challenged then-Sen. Biden in a Capitol Hill hearing on bankruptcy law, has noted repeatedly that they have sharply diverging viewpoints. Her standard campaign pitch doesn't mention Biden but is built around an assertion that the "time for small ideas is over," an implicit criticism of more moderate Democrats who want, for example, a public option health care plan instead of single-payer or who want to repeal Trump's 2017 tax cuts but not necessarily raise taxes further.
Biden, likewise, doesn't often mention Warren or Sanders. But he regularly contrasts the price tag of his public option insurance proposal to the single-payer system that Warren and Sanders back. In a pre-debate briefing, Biden campaign officials said he would reject the premise that he's an incrementalist, either in his long career as a senator and vice president or in his proposals for a would-be presidency. In an apparent rebuke of Warren, known for her policy plans, the advisers said Biden will hit on the idea that "we need more than just plans, we need action, we need progress."
Health care will top the list of examples, they said. They note that Biden's proposal for a government-run "public option" to compete with private insurance still would be a major market shift, even if it stops short of Sanders' and Warren's proposal for a government insurance system that would effectively end the existing private insurance system. And, by extension, the difference may make Biden's plan more likely to make it through Congress, they contend.
There are indirect avenues to chipping away at Biden's advantages, said Democratic consultant Karen Finney, who advised Hillary Clinton in 2016. Finney noted Biden's consistent polling advantages on the question of which Democrat can defeat Trump.
A Washington Post-ABC poll this week found that among Democrats and Democratic-leaning voters, 45% thought Biden had the best chance to beat Trump, though just 24% identified him as the "best president for the country" among the primary field.
"That puts pressure on the others to explain how they can beat Trump," Finney said. Voters, she said, "want to see presidents on that stage," and Biden, as a known quantity, already reaches that threshold. "If you're going to beat him, you have to make your case."
Harris, said spokesman Ian Sams, will "make the connection between (Trump's) hatred and division and our inability to get things done for the country." Buttigieg, meanwhile, will have an opportunity to use his argument for generational change as an indirect attack on the top tier. The mayor is 37. Biden, Sanders and Warren are 76, 78 and 70, respectively — hardly a contrast to the 73-year-old Trump.
There's also potential home state drama with two Texans in the race. O'Rourke and Castro clashed in an earlier debate over immigration. Castro has led the left flank on the issue with a proposal to decriminalize border crossings.
For O'Rourke, it will be the first debate since a massacre in his hometown of El Paso prompted him to overhaul his campaign into a forceful call for sweeping gun restrictions. O'Rourke has recently used the F-word in cable television interviews. He's given no indication whether he'll bring that rhetorical flourish to broadcast television.
Peoples reported from Washington.