Correction: An earlier version of this post noted that at the time of the experiment, CREDO members were promoting the “Save America's Pollinators Act,” introduced by Reps. Earl Blumenauer of Oregon and John Conyers of Michigan. The study’s authors said that was not the bill that was used in their study.

It’s a widely accepted truism in Washington: Campaign donations buy access.

While that belief governs much about the way politics operate, there’s a surprising lack of scientific evidence to bolster that assumption, which is the subject of substantial academic debate.

Two political science graduate students are now seeking to bring some precision to that discussion through the kind of randomized, controlled study used to test the impact of pharmaceuticals.

Joshua Kalla at Yale University and David Broockman at the University of California, Berkeley, are out today with the results of a novel field experiment that measured how campaign donations – even the prospect of them – alter the behavior of members of Congress and their staff.

To do so, they recruited the help of a real political group, the liberal organization CREDO Action, and embedded the experiment into a real lobbying effort during last summer’s August recess, when the group sought to secure co-sponsors for a chemical-banning bill.

Here’s how it worked: Last summer, a group of CREDO fellows e-mailed congressional offices seeking meetings to discuss the measure, sending one of two different form letters.

The first e-mail had the subject line: “Meeting with local campaign donors about cosponsoring bill.” The body of the e-mail said that about a dozen CREDO members “who are active political donors” were interested in meeting with the member of Congress in his or her home district to discuss the legislation.

The second e-mail stripped out the donor references and instead said “local constituents” were looking to meet the member of Congress.

In both cases, CREDO organizers noted that if a House member was not available, the group sought to meet with the most senior staffer available.

The e-mails went out to 191 members of Congress – all members of the same political party – who had not already co-sponsored the bill. (The study’s authors do not disclose which party the members represented, but it’s safe to assume they were Democrats, considering CREDO’s political orientation.) Each office was randomly assigned one of the two e-mails, with about two-thirds getting the request from constituents and one-third getting the request from donors.

It’s worth noting that all those who met with congressional offices were real CREDO members or political donors, none of whom knew they were part of an experiment.

The results: Only 2.4 percent of the offices made the member of Congress or chief of staff available when they believed those attending were just constituents, but 12.5 percent did when they were told the attendees were political donors.

Also, nearly one in five of the donor groups got access to a senior staffer, while just 5.5 percent of the constituent groups did. That means the donors had more than three times the access to top staffers than the constituents.

Broockman said he was surprised by the size of the difference, and noted that the study may actually underestimate the access of political donors, since none of the offices were told ahead of time the identities of the contributors, how much they had given – or even whether they had donated to that member of Congress.

That was done to address a question raised in the Supreme Court’s Citizens United decision, which argued that lawmakers are not influenced by political money that does not go directly to their campaigns.

“That was a really key piece,” Broockman said. “It gets very far away from the quid pro quos that the court suggested are the only ways influence operates.”

The study, currently under consideration for publication, drew praise from Donald P. Green, a professor of political science at Columbia University and expert in the use of field experimentation to study politics, who reviewed the results and said they showed a small but clear pattern.

“It’s convincing, but not overwhelming,” said Green, adding that he is eager to see others attempt to replicate the study in other arenas.

He praised the rigor of the experiment, saying such a randomized field study was long overdue.

“What is interesting is that it was done on the left, so it cannot be dismissed as a cheap shot at the right-of-center donors who are so often in the news,” said Green, who taught both Kalla and Broockman as undergraduates at Yale.

Becky Bond, political director for CREDO, which frequently participates in social science experiments, said she is not worried that the group will now have a harder time getting access to members of Congress who might be put off by being used as guinea pigs.

“We are really committed to having science have an impact on all of these discussions,” she said. “I have concerns that we don't have more of these experiments that help tell us the impact of money and politics.”