In the 2018 midterms, Democrats won big because they assembled a broad coalition: progressives, moderate Democrats, independents and even some Republicans who were disgusted by Donald Trump. That coalition turned out for Super Tuesday — and it turned out for Joe Biden.

That’s the coalition that can beat Trump.

It is now likely that Biden will be the Democratic nominee. But in order to unify the winning alliance that Democrats need to defeat Trump in November, Biden needs to move quickly to reach out to disappointed progressives who have been backing Elizabeth Warren, while also extending an olive branch to Bernie Sanders supporters.

There’s a simple way for Biden to unify the party: He should announce his pick for vice president this week.

Given that Biden is 77 years old, his pick for vice president matters more than most. But it’s also crucial politically. It would be Biden’s best way of signaling that he wants to be the candidate for everyone, not just those who have a mind meld with him ideologically.

One name immediately comes to mind: California Sen. Kamala D. Harris.

To build a successful coalition, the candidates on the ballot should reflect those who are voting. The Democratic party and the broader spectrum of voters needed to beat Trump is racially diverse, disproportionately female and mostly younger than Biden. The ticket should be, too.

Harris has distinguished herself as a formidable opponent of the Trump administration. In hearings, her questioning of Trump acolytes has been piercing and devastating. And on the debate stage, she knows how to skewer an opponent (just ask Joe Biden). Plus, by picking someone who criticized him harshly in the past, Biden would be showing a stark contrast with Trump, who demands unquestioning loyalty from the sycophants in the White House. And at 55, Harris balances out Biden nicely on the age front, too.

Use the Post Opinions Simulator to pick a state and see what might happen in upcoming primaries and caucuses.

There are other intriguing vice-presidential prospects, of course. Warren would have been an excellent president. She would make an excellent vice president. But she comes with the important caveat that if she left the Senate, her vacant Senate seat would be filled by the sitting Republican governor, Charlie Baker. That could swing control of the Senate to Republicans during the crucial first phase of a new Democratic administration. Democrats could be better served, perhaps, by having Warren appointed to a high-profile Cabinet post once it is clear that her Senate seat won’t be the deciding vote. Yes, that would be yet another annoying and unfair wait for Warren — but if there’s one thing we know about Warren, it’s that she persisted.

Stacey Abrams could also be a smart pick. As a rising star in the party, she would fire up the Democratic base while also putting a Democratic victory in Georgia within reach. Her critics would say that her experience in Georgia state politics hasn’t prepared her for the national stage, but any Biden ticket is going to present plenty of experience to voters.

Biden himself has been relatively coy about his plans for a potential running mate. He has said that his experience serving as President Barack Obama’s vice president made it clear to him that it was crucial to have a No. 2 who aligned with him philosophically. That would rule out someone like Sanders or Julián Castro, who are much further left of Biden.

If Biden wants to pick someone philosophically similar, he would likely favor Minnesota Sen. Amy Klobuchar, former Texas congressman Beto O’Rourke or former South Bend, Ind., mayor Pete Buttigieg. They are all possibilities. Klobuchar would help Biden in the Midwest. O’Rourke could help make Texas competitive. (A recent poll showed Biden leading Trump by one percentage point in the seemingly solid red state.) And 38-year-old Mayor Pete would bring military experience while counterbalancing the narrative of a graying campaign.

One mistake Biden should avoid is picking someone whom he has to introduce to the country. Sen. Tim Kaine of Virginia did a fine job as Hillary Clinton’s running mate in 2016, but he didn’t bring a lot of new excitement or a proven base of voters to the ticket. By contrast, Harris, Abrams, Warren, Klobuchar, Buttigieg and O’Rourke are already household names to most Democrats. Picking them would broaden Biden’s support, bringing core constituencies into a ticket that needs every vote it can find to beat Trump.

Biden won big on Tuesday. Now, he should name his running mate. The sooner Democrats coalesce around a nominee and a big-tent ticket, the sooner they can shift their focus to what really matters: beating Trump.

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