In measuring several different messages of various candidates without their names attached,
Greenberg told Biden's team that Warren's message about a “rigged” system was “dominant” among the groups Biden needs, such as millennials who may feel disillusioned by the tumult of recent decades.

Greenberg's analysis may be a surprise given that he is not exactly a left-wing firebrand. His most famous political clients include former British Prime Minister Tony Blair, Al Gore and Bill Clinton. He co-founded a firm with Democratic strategist James Carville and also co-authored a book with him in 2012. He has long criticized the Democratic Party on immigration, an appraisal he shared with Hillary Clinton, as she recounted in her post-2016 campaign book, "What Happened."

“Stan also thought my campaign was too upbeat on the economy, too liberal on immigration, and not vocal enough about trade,” she wrote.

Greenberg’s polling is not an outlier. A POLITICO/Morning Consult poll released this week showed Warren boosting Biden’s candidacy the most of any potential choice, particularly among people under 45 and black and Hispanic voters. And an April USA Today/Suffolk poll similarly found that 22 percent of Sanders voters would vote for a third-party candidate, Trump, or not vote at all.

“Above all else, [the Democratic Party] needs consolidation. That’s where the overwhelming percentage of votes are,” Greenberg argued.

In his presentation, which was titled "The obvious solution," Greenberg wrote, “The biggest threat to Democrats in 2020 is the lack of support and disengagement of millennials and the fragmentation of non-Biden primary voters.”

The Biden campaign declined to comment on Greenberg’s presentation, and Greenberg wouldn't say how it was received. ("Not off the record, either," he added).

Biden has recently begun vetting several women for vice president. Some allies say three of his former rivals — Sens. Amy Klobuchar, Kamala Harris and Warren — have an edge because of their experience running for president themselves. He is also considering New Hampshire Sen. Maggie Hassan, New Mexico Gov. Michelle Lujan Grisham, and Florida Rep. Val Demings.

Former President Barack Obama, who has been advising Biden on the choice, also thought it was very important to pick someone who had already gone through the rigors of the presidential campaign trail. “There’s not going to be a lot of time to adjust,” Obama said at the time, according to former adviser David Axelrod’s book, "Believer." “I’m afraid that if someone is experiencing this whole, crazy circus for the first time, it would be too much to ask.”

Greenberg’s argument that Warren would be the shrewdest choice politically cuts against much of the conventional wisdom, which holds that she may be a capable governing partner but does not bring much political upside. Among the raps against her: Her left-wing positions could alienate suburban voters the party needs, and Trump would have a field day going after her.

Many progressives have also argued that Biden should pick a black woman to help increase turnout among African American voters, particularly in Detroit, Philadelphia and Milwaukee. Even some of Warren’s allies acknowledge that just because her message may resonate doesn’t necessarily mean that she will. She’s not the nominee, after all, and she failed to place first or second in any 2020 primary, including her home state of Massachusetts.

But Greenberg is unmoved.

“I think the data is clear,” he said.

Greenberg has never worked for Warren. He got to know her when they attended a handful of policy dinners with his wife, Rep. Rosa DeLauro (D-Conn.), before the Great Recession more than a decade ago.

Greenberg also argues that there isn't evidence that someone like Harris would obviously increase African American turnout more than Warren would.

"If you look at the African American vote in the primaries, they weren't supporting Harris," he said. "She never developed the base in the African American community against Biden." The POLITICO/Morning Consult/ survey also showed Warren polling better among black and Hispanic voters than any other contender on the question of whether her selection would make them more likely to vote for Biden.

Underlying Greenberg’s case for Warren, however, is a more provocative analysis: that Democrats have a natural advantage as long as the party isn't splintered and that Trump is running the GOP into the ground. He made that case in his recently published book, "R.I.P GOP."

“The year 2020 will produce a second blue wave on at least the scale of the first in 2018 and finally will crash and shatter the Republican Party that was consumed by the ill-begotten battle to stop the New America from governing,” he wrote in the book.

Many Democratic operatives disagree with Greenberg and think Trump — with his gigantic war chest and knack for survival — might well win reelection.

Yet even as the global pandemic has upended the campaign, Greenberg said Democrats are still on track to win big if the party is united.

In "every trend [and in] every election since '16," he said, "the anti-Trump vote has played out stronger and stronger.”