In announcing her 2020 exploratory committee, Sen. Elizabeth Warren of Massachusetts railed against corporate influence in policymaking.

“Our government has been bought and paid for by a bunch of billionaires and giant corporations that think they get to dictate the rules that affect everyone,” she wrote in an email to supporters.

“Tax loopholes. Prescription drug pricing. Financial rules. Environmental protection. These companies define policies that are great for their bottom line, while good, honest people who work hard get squeezed harder every year. It’s corruption, pure and simple.”

Progressive lawmakers such as Warren (D) and Sen. Bernie Sanders (I-Vt.) have long been critical of the relationship between members of Congress and the business interests who lobby them. Last year, a study published in the American Political Science Review painted a stark picture of how corporations have skewed Congress’s perception of what the American public wants.

The study, by Alexander Hertel-Fernandez of Columbia University and Matto Mildenberger and Leah C. Stokes of the University of California at Santa Barbara, found that congressional staffers were terrible at estimating constituents' support for various policies. On nearly every issue surveyed — from gun control to environmental regulation to Obamacare — staffers in both parties believed the public was more conservative than they actually are.

The reason? Congressional offices pay a lot of attention to what big business groups are saying — especially when those groups give them money.

Well over half of GOP staffers said that input from business groups was “very or extremely important” in shaping the policy advice they gave their bosses, the study found. Over a quarter of Democratic staffers said the same.

“Staffer contact with these organizations may skew their perceptions of the public because the groups are more ideologically conservative than the public along the range of issues we study,” the researchers wrote.

Many staffers admitted changing their minds about a policy topic after speaking with a lobbying group that had given money to their boss’s campaign. “45% of staffers agreed they had developed a new perspective about a policy after speaking with a group that provided campaign contributions to their Member,” the authors write. “This result suggests that a potentially large share of senior Congressional staffers have had their thinking about policy shaped by interest groups that provided their Member campaign contributions.”

And 62 percent of legislative staffers said they would view letters from employees of a hypothetical large company in their district as representative of public opinion, versus 32 percent who said the same about letters from ordinary “constituents” not identifying any business affiliation. Perhaps most strikingly, 1 in 10 staffers told the researchers letters from ordinary citizens were “not at all representative” of public opinion in their districts.

“I think it’s an outstanding paper,” said Brendan Nyhan, a University of Michigan political scientist who was not involved with the study. “The article avoids simplistic accounts of money buying votes and instead provides evidence that representational biases may originate in part from information provided by contributors and/or communication from business groups.”

That representational bias may partly explain why Congress has often been willing to advance pro-business legislation even in the face of strong opposition from the public.

Congress passed an unpopular tax cut for corporations, for instance, while ignoring the public’s strong desire for an increase in taxes on the wealthy. It has dragged its feet on federal minimum wage increases. Lawmakers have worked to undermine unions, even as research increasingly suggests they’re a key to middle-class prosperity. The White House has been dismantling environmental regulations at a time when the public wants those regulations to be even stronger. Many Democrats in Congress are skittish about supporting Medicare-for-all at a time when 70 percent of Americans tell pollsters they want it.

The progressive wing of the Democratic Party has grown increasingly vocal about these issues in the first two years of the Trump administration. Warren evidently hopes to capitalize on this discontent.