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Senator Bernie Sanders introduced a radical bill last week that would overhaul America's health-insurance system. The Medicare for All Act of 2017 would turn the American government into the country's only payer—besides individuals—for health care. While the bill has virtually no chance of passing the Republican-controlled Congress, it's still symbolically important because it shows the Democrats' shift to the left on health care. Case in point: The last time Sanders (I-Vermont) introduced a similar bill, in 2013, he wasn't able to garner a single co-sponsor; this time, 16 Democratic senators signed on as co-sponsors.

What's driving this change of heart? According to University of Southern California political scientist Christian Grose, many of these co-signers are thought to have presidential ambitions and therefore may be "trying to peel off some support from Bernie Sanders voters were they to run in 2020," he writes in an email. In addition, the number of Americans who say they want to have a single-payer health-care system is growing, although they remain in the minority.

Might other interests be at work as well? MapLight, a non-profit that tracks how campaign donations affect politicians' voting, ran the numbers to see how much accident- and health-insurance companies have donated to sitting senators in their most recent election campaigns. Pacific Standard re-ran MapLight's analysis using the organization's "Find Contributions" tool and came up with similar results: On average, senators who didn't sign Medicare for All received about $48,000 from these industries between November of 2010 and November of 2016. That's nearly twice as much as the 17 senators who signed it, who received an average of $27,000. Democratic senators who didn't sign on received an average of more than twice as much in donations as Sanders and his 16 co-signers: $56,500. You can see our full results below:

Of course, these numbers don't show the whole picture. It's very difficult to track all the ways a company can donate to a political candidate. MapLight analysts sought to identify donations from companies, company employees, and relevant political action committees, but they may have missed some major contributions. "The money is more than likely grossly undercounted," says Paul Jorgensen, a political scientist who studies campaign finance at the University of Texas–Rio Grande Valley and is not involved with MapLight. "What it may not account for are the other committees that those sitting senators will be a part of that receive cash. The committee system in Congress is very complicated and the flow of money is very complicated. Just looking at the primary campaign committee is not sufficient for looking at an aggregate of campaign donations." Candidates may receive support from super political action committees, or super PACs, run by their party leadership, for example, which wouldn't show up in the data MapLight uses, which is drawn from the Center for Responsive Politics.

It's possible that the ratios we found—twice as much money given to Medicare for All's non-supporters—might be different in a full accounting of donations. Still, nearly all of the political scientists Pacific Standard consulted thought it was reasonable to report these numbers as a rough estimate of industry money involved in the Senate's votes on health care. They also said the ratios are plausible and, if correct, significant. (Jorgensen was the exception because he thought the undercount might be so severe.)

These numbers don't prove that industry donations made senators not support Medicare for All. To argue that, analysts would have to rule out other reasons senators may not support the bill, such as their personal ideologies or the desires of their constituents. Plus, the line of reasoning may run the other way. "It's just as likely that these senators were openly unfriendly to legislation such as the Sanders bill and that that is why the industry donated to them in the first place," George Mason University political scientist Jennifer Nicoll Victor writes in an email. "The reason for the donation isn't to 'corrupt' the senator or to buy their position on a bill, rather the donation is more like an expression of commonality, friendship, or alliance because they already agree with one another."

There's other evidence that shows money influences votes, at least for other bills. Jorgensen himself worked on a white paper for the Roosevelt Institute, a think tank that supports stricter regulation on business, that linked industry donations to congressional votes on the Dodd-Frank Act. MapLight's analysis isn't set up to make that same argument for Sanders' Medicare for All Act. MapLight spokesman Alec Saslow admits as much, but adds, "At a minimum, the outsized influence of money in politics gives the public reasonable cause to question our lawmakers' motivations."