Elizabeth Warren dropped out of the Democratic primaries on March 5. Bernie Sanders suspended his campaign a month later, on April 8. Yet, more than six weeks later, both Warren and Sanders continue to rack up votes in the primaries. In the Oregon primary, on May 19, Joe Biden got 66 percent of the vote, Sanders 20 percent, and Warren nearly 10 percent. In the Hawaii primary, which ended on May 22, Biden got 63 percent and Sanders nearly 37 percent.

Sanders, of course, is encouraging his voters to continue voting for him, so they can be represented by delegates who will shape the platform. But it remains astounding that a third of the Democratic Party continues to vote for someone other than Biden even after his receiving the endorsement of all his main rivals, including Sanders and Warren. By contrast, Donald Trump has a unified Republican Party behind him.

From the ongoing primary votes, it’s clear that even though Biden has the majority of the party on his side, there is a significant minority, about a third of voters, who are to his left. As he prepares for the general campaign, Biden’s main task has to be to unite the party by figuring out a way to get the voters who preferred Sanders or Warren to join his ranks. This is something that goes beyond merely voting in November. Sanders and Warren had the most enthusiastic and energized voters, the ones most likely to donate to and canvass for their candidates. Biden needs those voters to feel they have a stake in not just defeating Trump but also in being active participants in the campaign.

Biden has been making policy overtures toward progressives, notably in selecting the committees that will shape the party platform. But he still has a way to go. In April, longtime Democratic Party strategist Stan Greenberg warned the Biden camp that Sanders voters were “dangerously not” ready to accept Biden yet. A recent Change Research poll in Michigan showed that Biden was doing 5 points worse among voters under 35 than Hillary Clinton did. Biden leads the state by 49 percent to Trump’s 46 percent. If Biden could get young voters as excited as they were in 2016, he’d have a much more comfortable lead in a crucial swing state.

A recent poll conducted by Data for Progress laid out a strong case that Warren is the candidate who is best positioned to unite the party if selected as a running mate. According to the polling organization, “Sixty-one percent of Sanders supporters would be more likely to vote for a Biden/Warren ticket, compared to 42 percent for a Biden/Harris ticket, 33 percent for a Biden/Klobuchar ticket and 26 for a Biden / Abrams ticket.”

Unifying the party goes beyond any simple polling advantage. The consensus among political scientists is that a vice-presidential candidate makes little difference unless they are notably unpopular (Sarah Palin probably cost John McCain between 1 and 2 percent). But a vice-presidential pick is important for building a coalition and getting different factions of a party to work together.

As David Leonhardt pointed out in The New York Times, many of the most successful presidential tickets have involved a balancing act between candidates who are dissimilar. “Donald Trump, a divorced reality-television star, chose a religious conservative,” Leonhardt notes. “Barack Obama and George W. Bush, both worried about seeming inexperienced, chose party elders. Ronald Reagan, who was labeled a radical conservative, chose an establishment figure: George H.W. Bush.” By contrast, Hillary Clinton’s pick of Tim Kaine did little to help her, since he shared her political profile.

The fact that Warren has a record of challenging Biden on issues like bankruptcy law might make him averse to picking her. But, in cold political terms, these earlier fights should make her a more attractive candidate, one whose selection can bring the party together. She has real credibility with progressives and can be trusted to be their advocate in a Biden administration. Even progressives disappointed by her backtracking on Medicare for All still know she’s by far the most left-wing candidate on Biden’s short list.

But Warren’s credibility goes beyond her policy prescriptions. She also has the specific character traits needed in a top-level government official. Trump’s corruption and incompetence have demoralized the civil service, notably the State Department and the Centers for Disease Control. Any successful Democratic administration will need to rebuild the administrative state, attracting thousands to join the new government. Warren, uniquely among the plausible picks to be Biden’s running make, has built up a network of policy wonks who could help fulfill this task.

As Jamelle Bouie of The New York Times notes,

It simply isn’t enough to write and pass a bill. You need experienced officials and agency heads, a fully staffed and well-seasoned federal bureaucracy and skilled political leadership to manage the entire operation. You need a Congress ready to adjust programs as needed and lawmakers skilled in oversight.

You need, in other words, state capacity—the ability to actually deliver on plans and mandates. And if there’s anyone in the Democratic Party who has thought deeply about the challenges of state capacity, administration and personnel, it’s Warren.

Warren has already proven her skills in building up the administrative state by being the political force behind the creation of the Consumer Financial Protection Bureau.

Biden will face the enormous task of cleaning up Trump’s mess, with both the pandemic and the economic meltdown requiring policies that are quick, bold, innovative, and far-reaching. It’s hard to think of anyone better equipped to help Biden in such a crisis than Warren. She’s the politically smart pick. She’s also the best person for the job.