Strategists working on the 2020 contest almost uniformly agree that no candidate can expect to amass predominant support from any group of voters in the first states on the calendar. “What is so fascinating about this race is a lot of people are going to be competing for the same spaces,” says Ben Tulchin, the pollster for Sanders. As a result, breadth of support might be more important for the contenders than depth: The candidates who emerge from the tangled pack in the race’s opening laps will likely need to piece together slivers of support from a wide variety of groups. “That’s the way the early states are going to be decided: who can tap into the most buckets, instead of the one that owns a whole lane to themselves,” said a senior adviser to Senator Amy Klobuchar, who, like some other advisers I spoke with, requested anonymity to discuss the race’s dynamics.

But as the field narrows, that equation could flip, many strategists believe. As the number of candidates shrinks, one of the critical variables is likely to be which survivors can consolidate the most support the fastest from the groups most drawn to them. And for all of the candidates already jostling in the race, relatively few alternatives might be able to compete with Biden for middle-aged, middle-of-the-road voters, particularly in the middle of the country.

This potential mismatch between the pools of voters and candidates looms so large because, even amid all of the party’s other demographic changes, older voters constitute a surprisingly large share of the Democratic primary electorate. Fully 60 percent of primary voters in 2016 were 45 or older, according to an analysis of all 27 exit polls that year conducted by the CNN polling director, Jennifer Agiesta. And while the Democratic primary electorate is growing more racially diverse, about two-thirds of those relatively older primary voters were white.

In the coming months, before the 2020 field is winnowed, it’s easy to envision multiple candidates targeting African American voters (Senator Kamala Harris, Senator Cory Booker, and Biden, with Sanders aiming at the youngest among them); Hispanic voters (O’Rourke, Biden, Sanders, and the former federal housing official Julián Castro); well-educated white women (Klobuchar, Harris, their fellow senators Elizabeth Warren and Kirsten Gillibrand, and maybe O’Rourke); activist liberals (Warren, Sanders, and, to some extent, Harris); and centrists (Biden, Klobuchar, O’Rourke, former Colorado Governor John Hickenlooper, and maybe Booker).

The competition might be especially spirited for younger voters. Sanders, despite his own age, dominated this demographic in 2016 with his crusty authenticity; he won more than 70 percent of those ages 18 to 34, according to the cumulative exit poll. With his mastery of social media, and his skill at framing choices more in terms of values than policies, O’Rourke in Texas displayed an electric capacity to mobilize young people: He carried more than 70 percent of voters under 30 in his Senate race. With that in mind, some Sanders supporters are already privately signaling that they are prepared to attack O’Rourke from the left, arguing that he’s too centrist for younger voters.