So far, the race has been chiefly defined by a central question: Should Democrats rally behind former Vice President Joseph R. Biden Jr., a moderate who is the field’s best-known candidate, or find a more progressive alternative. While Mr. Biden has proved to be resilient in the polls since entering the race in April, thanks in large part to his appeal with older and moderate Democrats, he is a fragile front-runner and has already seen his advantage ebb in Iowa and New Hampshire.
Senator Bernie Sanders of Vermont has retained much of the grass-roots and financial network that powered him to unexpected success in the 2016 Democratic race, but he has struggled to expand his appeal beyond his committed supporters.
That is in part because the party’s left flank now has a wealth of alternatives, including Ms. Warren, who has recently surged in a number of surveys after months of laying out a series of ambitious policy proposals. But not all of these candidates will be on the same stage this week.
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For Wednesday’s forum, Ms. Warren loomed well above the other nine candidates flanking her on either side. She has gained considerable strength as a champion for the party’s progressive wing, stitching together a still-developing coalition heavy on young people, women and educated liberals. In some national and early-state polls, Ms. Warren has caught up with Mr. Sanders as the second-place challenger to Mr. Biden, or come close to doing so.
Yet Ms. Warren still faces skepticism from influential constituencies in the party, particularly about her ability to win the general election. The debates offered her a chance both to show Democratic voters that she is capable of defending her liberal policy proposals before a wide audience, and to project the kind of flinty resilience that might give Democrats confidence she can hold her own against a president who has reveled in attacking her personally.
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For many — perhaps most — of the other candidates on stage on Wednesday, the debate appeared to be less a test of momentum than a bid for relevance. No other candidate has broken the 5-percent mark in recent Democratic primary polls, and at least one, Mr. O’Rourke, has seen his support fade steadily since the beginning of the year.
Mr. Trump’s pledge to begin a mass roundup of families residing illegally in the United States had the potential to thrust immigration even more squarely into the center of the Democratic primary contest. Several of the candidates used their trip to South Florida to visit a local detention center for unaccompanied migrant children.
“What is happening here in Homestead is wrong,” Ms. Warren said outside the detention center on Wednesday, hours before the debate. “And we will fight it with everything we have.”