The field of 2020 Democratic presidential candidates has finally been whittled down by the Democratic National Committee to 10 qualifiers for tonight’s three-hour televised debate in Houston.

The debate will be the first time the three leaders in the polls — former Vice President Joe Biden and Sens. Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont — will be on the same stage, along with seven other survivors of the original field of 25.

They include Sens. Kamala Harris of California, Cory Booker of New Jersey and Amy Klobuchar of Minnesota; South Bend, Ind., Mayor Pete Buttigieg; former Texas Rep. Beto O’Rourke; former Obama Housing and Urban Development Secretary Julian Castro; and technology executive Andrew Yang.

Warren and Sanders will be vying for identity as the principal progressive candidate against moderate liberal Biden, while the others, polling in single digits, hope to capture enough political oxygen to continue on through the early state caucuses and primaries.

Warren has shown considerable growth in the polls at Sanders’ expense, as he clings to the support he surprisingly built in his failed challenge to Hillary Clinton in 2016.

But Biden has had the early advantage of being the best-known Democrat with the most political experience in the field. He also benefits from Sanders and Warren splitting the progressive field that seeks to move the party past the Bill Clinton and Barack Obama years.

The former vice president, however, has had to endure sniping for past inconsistencies in his personal narrative, in which he has asked voters to accept “my word as a Biden” on his dependability. It raises questions, at his age of 76, of whether he is getting too old to function at a high level of efficiency as president.

It might be expected that the rest of the still-large Democratic field would form a circular execution squad in the debate to reduce Biden’s early lead to their mutual political advantage. Biden’s performances in the first two debates this year have been described as lackluster and reflective of earlier days for a party that should have its eyes on the future.

But with a Republican president widely regarded in Democratic circles as unstable and corrupt, it’s not surprising that Biden would be seen among many voters as the logical vehicle for returning to the cherished norms of demeanor of past administrations of both parties. His record and his pitch for working across the partisan aisle can have particular voter appeal in the Donald Trump era of purposeful disorder.

So far, Warren’s upbeat and detailed policy approach has been the surprise of the early campaign. For all the talk about electability as a key argument for Biden’s nomination, recent polls have suggested that all of the leading Democrats in the race would be favored to defeat Donald Trump if the election were held now.

The media focus going into tonight’s debate will be on Warren and Biden, and whether Warren will seize on Biden’s defense of his past and particularly his role in the Obama administration.

In New York last week, Biden referred to Warren’s detailed plans for the country by alluding to his own executive experience, observing: “You have to have plans, but you have to be able to execute those plans.”

At a New Hampshire state Democratic convention on Labor Day, when she was asked to compare her campaign with Biden’s, Warren demurred. “All I can do is stand up and talk about why I’m running. I’m not here to criticize any other Democrat or anyone else’s campaign.”

One key policy difference between them is the fate of the Affordable Care Act, the Obama administration’s health insurance plan that Biden helped guide through Congress against repeated Republican efforts to kill it. Warren favors Sanders’ “Medicare for All” plan that would replace it. Biden supports building on Obamacare by providing coverage of pre-existing conditions and allowing holders to keep private policies on which employers or trade unions pay the premiums.

Tonight’s debate should at least shed more light on this basic difference between the candidates, and it could enable others to gain the traction with voters that they’ve lacked so far.

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Jules Witcover’s latest book is

“The American Vice Presidency: From Irrelevance to Power,” published by Smithsonian Books.