It’s a truism of modern politics that Congress is not particularly responsive to the needs of its middle- and working-class constituents.

The drivers of that dysfunction are legion, including partisan gridlock, electoral gamesmanship and even the structural elements of American democracy itself. But new research points to an overlooked reason Congress is often seen as out of touch with the needs of the non-wealthy: the decline of union membership.

Researchers Michael Becher at the Institute for Advanced Study in Toulouse and Daniel Stegmueller at Duke University study political inequality, or the well-documented tendency among lawmakers to favor the interests of their wealthiest constituents. “The preferences of the poor seem to be virtually ignored,” as the authors characterize the existing research.

In recent years, for instance, Congress has slashed taxes on the wealthiest Americans but done little to help the poorest — such as raising the minimum wage or implementing a true universal health-care system.

That political inequality is both a cause and a consequence of the nation’s runaway income and wealth inequality: The rich become richer, giving them more leverage to influence Congress, which in turn passes legislation that help the rich amass even more wealth. Many economists think the remedy is relatively simple: Redistribute the money, typically via the tax code. But history suggests that’s unlikely to happen.

Given this reality, “it may appear that unequal democracy is an inherent feature of capitalism,” Becher and Stegmueller write. But they’ve compiled evidence that unions, which help bolster the economic fortunes of the working class relative to the wealthy, could similarly be a countervailing force against political inequality.

They began by assembling a massive data set, comprising 223,000 survey responses to the Cooperative Congressional Election Study from 2006 to 2012, then adjusted it using 3 million individual Census American Community Survey responses to generate a representative sample for each U.S. House district.

The CCES asked respondents whether they would support more than two dozen policies corresponding to House roll call votes during that period. “These cover important legislative decisions, such as Dodd-Frank, the Affordable Care Act (and attempts to repeal it), the minimum wage increase, the ratification of the Central America Free Trade Agreement, or the Lilly Ledbetter Fair Pay Act,” the authors write.

They then broke out preferences for these policies by income group: people in the top, middle and bottom third of income distribution in each House district.

They calculated union membership in each district using public administrative records, and obtained House votes for the policy polled by the CCES. With this data in hand, they turned to their central question: How does union strength in a given district affect members’ voting patterns?

They found, first of all, that lawmakers are much more responsive to their high-income constituents.

“A [standard deviation] increase in the preferences of individuals in the upper-third of the of income distribution is linked to an increase in the probability of legislators to cast a corresponding vote of 13.6 percentage points,” the authors write. “In contrast, a [standard deviation] increase in the preferences of those in the bottom-third induces a much smaller change in legislators’ behavior of 1.6 points.”

However, a strong union presence altered the dynamic. A standard deviation increase in union membership, for instance, “increases legislative responsiveness towards the poor by about six to eight percentage points,” they write. “As a result, in districts with relatively strong unions legislators are about equally responsive to rich and poor Americans.”

Here’s another way to think about it: A move from a district with a median level of union membership to one in the top 25 percent “increases the responsiveness of legislators to low-income preferences by eight percentage points, while it decreases responsiveness to high-income preferences by about five points,” the researchers write.

They presented evidence that this relationship is more than just a correlation, controlling for a number of factors that also affect legislative responsiveness, such as state-level union policy, demographics and social capital.

They also used the geography of union membership as an instrumental variable to rule out confounding factors. “Economic historians have noted that in the early 1950s coal and metal mines as well as steel plants were fully unionized across the country, irrespective of local political leanings,” they noted. The location of those plants was largely determined by geography — the presence of raw materials in the earth — rather than by local social, political or human factors.

Rather than ask “how do increases in union membership affect legislative responsiveness,” they were able to ask how “increases in union membership due solely to geography affect legislative responsiveness.”

The additional analyses reaffirmed their main findings and “strengthen the causal interpretation of our results,” they write.

Overall, the analysis suggests that higher union membership would help combat not only economic inequality, but also political inequality by making legislators more responsive to the needs of their poor and middle-class constituents.

Conversely, efforts to weaken unions, as undertaken by Republican lawmakers across the country, would make legislators even less accountable to the working class.

In the end, the social science literature suggests that there are numerous ways out of the morass of legislative failure the country has recently found itself in, ranging from redistributive tax policy to the bolstering of unions. But those solutions involve shifting money and power away from the privileged institutions and institutions that have spent the past half-century tightening their grip.