Potential Democratic presidential candidates in 2020 are already under the microscope.

Did Kamala Harris cozy up to the Israel lobby by speaking "off record" to AIPAC’s annual conference in Washington last week? (Sounds like it, according to The Intercept.)

Did Mark Warner sell out to Wall Street by voting to water down Dodd-Frank bank regulations? (No: Warner, a tech entrepreneur, has always had a Wall Street soul.)

Did Kirsten Gillibrand throw Al Franken under the bus? (Maybe not, but I know some women in Minnesota who won’t vote for her.)

But what the candidates stand for (and against) is arguably less important at this early juncture than the campaign strategies they choose to pursue. Here the choices facing Democratic voters are starker and simpler, driven less by personality and policy than by political calculation and the urgency of (re)defeating Trump.

Lessons of 2016

The results of the 2016 election do not provide an obvious answer.

In the general election, Hillary Clinton ran to the center, appealed to independents and suburban women, and won the popular vote—but not in the swing states. So she failed to win the presidency. In the Democratic primaries, Bernie Sanders ran to the left and ignited a social movement—yet he failed to win the party's nomination. So we don’t know if his approach could have prevailed in a general election.

Sanders’ supporters can blame his loss on Clinton’s control of the Democratic National Committee, which surely hurt him. But Clinton won a total of 3.6 million more votes, indicating that Sanders' loss was not caused by Debbie Wasserman Schultz alone. Sanders’ appeal was critically limited, especially among older African Americans.

But a massive new voter study provides empirical data suggesting that the Sanders strategy is likely to be more effective for Democrats in 2020—with one caveat.

Demographic Future

The study was done by Sean McElwee, a co-founder of Data for Progress, and three academic political scientists: Jesse H. Rhodes and Brian F. Schaffner of the University of Massachusetts, Amherst and Bernard L. Fraga of Indiana University.

They studied the ballots of 64,000 voters in 2012 and 2016 and came away with a key finding: The number of voters who cast a ballot for Obama in 2012 and did not vote in 2016, or voted for a third-party candidate, outnumbered those Obama voters who pulled the lever for Trump.

This bloc of voters, they argue in a piece for the New York Times, is the key Democratic constituency in 2020.

Obama-to-nonvoters are a relatively liberal segment of the country who have largely been ignored. They are mostly young and nonwhite, and they represent an important part of the Democratic Party’s demographic future. Given the likelihood that many Obama-to-Trump voters will remain in Republican ranks, it is hard to imagine how Democrats can win elections if this group remains on the sidelines.

To win the Obama-to-nonvoters bloc, the authors of the study say Democrats need to understand who these voters are.

Fifty-one percent were people of color (compared with 16 percent of Obama-to-Trump voters and 34 percent of Obama-to-Clinton voters).

Twenty-three percent of Obama-to-nonvoters were under 30 (compared with 11 percent of Obama-to-Trump voters and 10 percent of Obama-to-Clinton voters).

More than 60 percent of Obama-to-nonvoters make less than $50,000 a year, compared with 45 percent of Obama-to-Clinton voters and 52 percent of Obama-to-Trump voters. That suggests the Democrats need a candidate who can win over young people of color who are not upper-middle-class.

But the study also pointed to a possible weakness in the “run to the left” strategy.

The Obama voters who stayed home, while generally liberal, were significantly less liberal on four of five key policy questions than the loyal Democratic voters who voted for both Obama and Clinton. In other words, more liberal policy positions were not sufficient to lure them to the polling booth.

What made the difference for these non-voters was personal contact, or the lack thereof.

“Only 43 percent of Obama-to-nonvoters reported being contacted by a candidate in 2016, compared with 66 percent of Obama-to-Clinton voters,” the authors say.

That would suggest one key to winning the non-voters is an aggressive get-out-the-vote effort. Indeed, a Democratic candidate who combines slightly more conservative policy positions with grassroots organizing, especially among African Americans (someone like Barack Obama), might have the best chance to win in 2020.

Whether that’s Harris, Warner, Gillibrand, or Sanders himself, remains to be seen.