Bernie Sanders has put the rest of the Democratic Party presidential primary candidates in a tough spot.

In the first six weeks of his campaign, Bernie has raised over $18 million. That money comes from more than half a million donors, with an average contribution of $20. He also has money left over from previous races, giving him about $28 million to work with.

It’s not that others in the crowded primary field can’t raise big money. Glitzy fundraisers and corporate schmoozing are second nature to Democrats. But the senator from Vermont has thrown them a curveball. By refusing to cozy up with titans of industry and relying instead on ordinary working people to fund both of his presidential campaigns, he’s turned Democrats’ most reliable fundraising strategies into a political liability.

Presidential hopefuls who lack Bernie’s sizable small-donor base are awkwardly contorting themselves to both meet their fundraising goals and win the favor of an increasingly progressive constituency. Senator Kamala Harris is a case in point, publicly touting her “real, grassroots campaign — by the people and for the people” while privately hitting the Democratic Party’s well-established big-donor circuit, making personal phone calls to billionaires and visiting the homes of major Hollywood executives. She has trumpeted the number of small contributions to her campaign thus far, but declined to say what percentage of her total haul they account for.

Senator Cory Booker, too, has assured the public that his “campaign is funded by grassroots donations from people like you.” But behind the scenes, Booker’s fundraising strategy is business as usual. Less than two weeks ago he raised $300,000 at the New Jersey governor’s mansion, an exclusive affair that featured a three-course meal and an appearance by Jon Bon Jovi. A week later, tech tycoons glad-handed Booker at a fundraiser attended by Silicon Valley billionaires. Booker has declined to release his fundraising totals — perhaps embarrassed by the low dollar count, or the low donor count, or both — but he’s been heartily embraced by extravagantly wealthy hotel magnates and hedge-fund executives.

In 2016 Bernie rejected corporate PAC money. The move played well with working people concerned about corporate influence in politics, and many Democrats have found it politically prudent to follow suit, including Harris and Booker.

But this presidential cycle, it’s becoming clear that officially rejecting corporate PAC donations is not synonymous with holding industry elites at arms’ length. While the widespread adoption of the practice may lead to decreased influence of corporate PACs themselves in the Democratic Party, it also functions as a means of dissimulation, allowing pro-business Democrats to strike a progressive pose while quietly maintaining bonds with wealthy individual donors.

Senator Kirsten Gillibrand declared that her campaign “is going to be run for and by people, not corporate PACs — that’s how we live by our values.” Gillibrand, who has taken millions of dollars from corporate PACs in the course of her career, clearly knows which way the wind is blowing.

But in addition to cozying up to Wall Street donors, she also recently held a fundraiser at the home of an executive at the pharmaceutical company Pfizer. Gillibrand has publicly declared her support for Medicare for All, which would allow the government to negotiate drug prices and erode the profits of the same pharmaceutical executives with whom Gillibrand is deliberately currying favor. Her support for Medicare for All can’t be trusted in light of her fundraising behavior — but rejecting corporate PAC money conveniently throws the public off her trail.

Senator Amy Klobuchar is also rumored to have been “particularly aggressive on the national donor circuit.” She has sworn off corporate PACs, which has helped her shore up a progressive reputation despite her business connections. Klobuchar’s campaign is a good example of the fragile balancing act Democrats are performing: in public she has been a harsh critic of tech giants, while in private she has maintained a close personal relationship with tech billionaire Sheryl Sandberg, and recently held an expensive fundraiser in San Francisco hosted by Silicon Valley angel investor Tom McInerney. Meanwhile, her social media presence is replete with references to her “homegrown” campaign.

Like the others, Mayor Pete Buttigieg has not shied away from expensive fundraisers hosted by economic elites. His otherwise impressive early donation haul of $7 million came from just 76,000 individuals, including some very wealthy donors. He has also said that he opposes boycotting companies for political reasons, and he pointedly did not endorse a ban on lobbyist donations to the DNC. But he has publicly stated he won’t be taking corporate PAC money, so his progressive bonafides may well remain intact to anyone not looking closely.

Clearly Harris, Booker, Gillibrand, Klobuchar, and Buttigieg’s rejection of corporate PAC money is not a solid gesture of allegiance to the working class in the fight against capitalist power. It’s a compromise they have to make in the face of a credible challenge from the left. That challenge is posed chiefly by Bernie Sanders, who both refuses corporate PAC money and refuses to solicit donations from industry magnates — and has fared all the better for it.

To her credit, Elizabeth Warren has refused to make that compromise. Like Bernie, she has taken a step beyond rejecting corporate PAC money, eschewing the donor circuit altogether. At the same time, she lacks something Bernie has in spades: a massive organic base of supporters who can provide a steady stream of small donations. Warren and Sanders share many of the same short-term policy objectives, but while Sanders’s appeal is inspirational, foretelling a mass movement of working people against billionaire power, Warren tends to rely on her policy expertise to sell her candidacy. For this reason, among others, she has failed to build a large and energetic base of supporters that can supplement the funds she’s lost by dropping out of the traditional fundraising game.

Warren’s financial director advised her to emulate the other Democratic Party candidates and solicit donations from rich people in private, but she has so far refused. As a result, her campaign funds are dwindling, and she may not be able to hold out much longer.

Since throwing his hat in the ring a few weeks ago, Beto O’Rourke has also announced his intention to decline corporate PAC money, and has so far held no fundraisers. If first-day donation hauls are any indication, O’Rourke’s base of individual donors is about half the size of Bernie’s — but still bigger than Warren’s, which means he might be able to skip the fundraisers and hold onto his grassroots image for a while.

But big donors love O’Rourke, and historically he has loved them back. Given his ambiguous political commitments and his conservative voting record, he may not be able to resist the siren call of the donor class.

And it is hard to resist, because there’s a lot of money sitting on the table. Politicians don’t have to pry donations from wealthy individuals: the capitalist class has its own reasons for infusing massive amounts of cash into elections. Political scientist Thomas Ferguson summed up the incentives for rich people to invest in campaigns this way: “In politics, you get what you pay for. Or someone else does.”

The rich have a vested interest in deregulating their industries, lowering their taxes, and otherwise protecting their profits — and they know that if they don’t pay to bend politics to their will, someone else will come out on top. That someone else might be a corporate competitor. It might even be the whole working class, the majority of people whose economic exploitation is the lifeblood of profit accumulation, which would be disastrous for capitalist elites.

Ultimately, the wealthy need to intervene in politics to maintain their dominance in society. The result is that any political party that isn’t explicitly for the working class and against the capitalist class will be awash in corporate money. The effect on actual politics is grim: where the public disagrees with corporations, Ferguson argues, corporate-aligned parties often conclude that it’s cheaper and easier to try to change public opinion than to snub elite donors with blank checks in hand.

The only kind of candidate who can hope to break with that pattern is someone who understands that the working class is in unavoidable conflict with the capitalist class, and who chooses the side of the workers every time.

And the only candidate who can hope to break with the pattern and to win is someone who inspires huge numbers of people not simply to elect them, but to join a mass movement that puts people over profit. As we often say on the Left: they have the money, but we have the people. But we also have the people’s money, if we can credibly convince the cash-strapped majority that a small donation will go a long way to securing a more equal and democratic future.

Bernie Sanders has these criteria on lock. It still may not be enough for him to clinch the presidency. But it is enough to send corporate Democrats into a panic as they attempt to mediate between their competing allegiances — on one hand to the majority of voters who are barely scraping by, and on the other to the tiny capitalist minority that exploits those voters for personal gain.