When I talk to Democrats about the presidential primary, I hear the same thing over and over: “I’ll vote for whoever I think can beat Donald Trump.”

Fair enough. The problem is that there’s no way to know for sure who that might be. The latest polls show all of the top Democratic Party presidential candidates beating Trump in a general election. But then, these same polls also showed Hillary Clinton beating Trump, so who can trust them? In order to figure out who has the best chance of beating Trump, we’ll need to reason it out.

Electorally speaking, Trump won for two related reasons: in key swing states, a handful of former Obama voters opted for Trump over Clinton, and another handful decided to vote for nobody at all. To unseat Trump, the nominee will have to perform well in those swing areas. Instead of projecting our own fantasies about what voters (and potential voters) are looking for in a candidate, we should look at the supporter data that is already available.

The Daily Beast reports that in the 206 counties that went for Obama in 2008 and 2012 and then went for Donald Trump in 2016, Bernie Sanders is raking in far and away the most individual donations. Sanders has received 81,841 donations from 33,185 donors in flipped counties. That’s roughly three times as much as runner-up Elizabeth Warren, who received 26,298 donations from 13,674 donors. Buttigieg comes in just under Warren, with Biden trailing closely behind.

It would be a mistake to draw a direct line from donors to voters. While it’s clear that Sanders has the most enthusiastic support base (he continues to break records for individual donations), we can’t just perform simple multiplication to predict the voter breakdown. Donor profiles differ from voter profiles in key ways: donors are more engaged in the political process, but plenty of semi-disengaged people will pull the lever in their state’s primary election, and plenty of even more disengaged people will do the same in the general election.

But we can still draw meaningful insights from this data. In particular, the fact that donors tend to be more politically engaged than non-donors makes the presence of passionate Sanders supporters in flipped counties especially important.

Each enthusiastic donor — especially someone donating this far in advance of the general election — is more likely than the average person to talk to their friends, family, and coworkers about politics. Early donors are likely to put bumper stickers on their cars and signs on their lawns. If they’re union members, they’re more likely than passive non-donors to take an interest in their union’s 2020 electoral work. They’re more likely to knock on doors and join voter registration drives. All of this will have a major impact on the political climate in their local areas come general election time.

And, importantly, the Sanders campaign has built a unique infrastructure to put its supporters to work. For years — but intensifying in 2019 — the campaign has been developing a “distributed” grassroots organizing network that mobilizes supporters to become volunteers themselves.

At Sanders supporter events, known as “barnstorms,” attendees are asked to volunteer to host a canvass or phonebank on the spot, and others are asked to sign up to participate in those events. The campaign has its own app that allows supporters to recruit people they meet into this network in real time, wherever they are. In late August, the campaign counted 11,000 campaign organizing events nationwide so far, the vast majority of which have have been led by volunteers, not campaign staff members. Sanders’s campaign even organized the Bernie Sanders Summer School, a boot camp for 1,500 student organizers who are prepared to act as boots on the ground on college campuses all over the country.

There are many additional points to be made about the content of Sanders’s campaign, and how his bold pro-worker message is likely to appeal to voters in swing states, particularly in the deindustrialized Midwest and Rust Belt. But those are for another article. On the level of basic electoral strategy, unseating Trump requires voters everywhere, especially in swing regions, turning out and pulling the lever for Trump’s opponent.

And they need to be convinced to do this not just by campaign staff, but by the people in their immediate social networks. The campaign best positioned to pull this off is the one that already boasts the most early and enthusiastic support from people who are dedicated to organizing in the nooks and crannies that official campaign staff can’t reach.

The candidate most likely to beat Trump is the one with the massive volunteer army, already hard at work all over the country, and especially in swing states and counties. That candidate is Bernie Sanders. No one else comes close.