Supreme Court Justice Elena Kagan, from left, Chief Justice John Roberts, Justice Anthony Kennedy, Justice Ruth Bader Ginsburg, Justice Stephen Breyer, and Justice Sonia Sotomayor arrive before President Barack Obama delivers the State of the Union address to a joint session of Congress on Capitol Hill in Washington, Tuesday, Jan. 12, 2016. (AP Photo/Evan Vucci)

Each week, In Theory takes on a big idea in the news and explores it from a range of perspectives. This week, we’re talking about corruption. Need a primer? Catch up here.

Zephyr Teachout is a law professor, author of the book “Corruption in America” and candidate for Congress in New York’s 19th District.

Members of Congress spend the majority of their time fundraising from wealthy donors, learning the smallest details about donors’ lives — at the expense of learning about the policy details most relevant to their legislative work. When they’re not fundraising, members may be anxious about meeting their fundraising quotas set by the national committees, or worried about offending the secret donors to powerful super PACs. This lurking fear undoubtedly shapes policy decisions, lest a wrong move trigger a deluge of attack ads from special interests.

The Supreme Court has said that none of this is corrupt or corrupting. That defies law, history and logic. In a recent case Chief Justice John Roberts wrote: “Any [campaign finance] regulation must instead target what we have called ‘quid pro quo’ corruption or its appearance. That Latin phrase captures the notion of a direct exchange of an official act for money.”

While quid pro quo is, in fact, a Latin phrase, that’s the extent of the rightness of his argument. The phrase comes from contract law, and traditionally was used to describe a relatively equal exchange between parties to a contract. It is not historically a phrase from corruption law. “Quid pro quo” appeared less than 100 times in all state and federal bribery and extortion cases before 1976. That year, Buckley v. Valeo struck down limits on campaign spending while upholding limits on campaign contributions. There, the Court used the phrase quid pro quo in passing. But the Roberts Court has clung to it, using it to narrow the definition of corruption and thus broaden the limits of what our representatives can do. This new standard is inappropriately limited and reveals an unrealistic view of the corruption in our politics.

Our founding generation understood that corruption happened whenever those in public power use public power for private ends. They also understood corruption as the central threat to the survival and flourishing of our country. During the Constitutional Convention, the Founding Fathers debated the corruption implications of dozens of Constitutional provisions, and George Mason described their job as protecting against corruption lest the country “be at an end.” Alexander Hamilton described the convention as a project in enacting “every practical obstacle to corruption and cabal.”

We know the Founders weren’t just talking about direct exchanges of money for official action, because as Professor Larry Lessig has shown, only five of the 325 mentions of “corruption” in the debates around the ratification of our Constitution referred to what would now be considered criminal bribery. The rest referred to instances where those in public power used that power for private, selfish ends. The fact that the narrow quid pro quo definition has replaced the traditional idea of corruption is not just bad history; it’s a dangerous misunderstanding.

The stakes are high. The quid pro quo understanding is vastly naïve, and Robert’s distortion of a critical term in our history has made a vocabulary fight into a constitutional crisis. It overturns laws that were designed to address truly dangerous big money forces.

Lobbyists have gained grotesque amounts of power in legislation, allowed to use campaign cash and the revolving door of political appointees to gain influence. According to the Court, as long as you don’t say magic words of exchange while talking to a lawmaker, your financial support is not corrupting, even if it sways legislation and policy. To add insult to injury, the Court is now — in McDonnell v. US — also considering narrowing traditional bribery laws as well, adding requirements before prosecutors can bring basic federal corruption charges. A tendency to constrain corruption in both campaign finance and criminal bribery law will lead citizens with few tools to fight a serious threat to our democratic culture.

It’s common sense that big money is corrupting: When corporations spend vast amounts of money, they do it because they they know that they can use that capital to exert their influence. A 2014 study by the Sunlight Foundation showed that $5.8 billion spent by corporations in lobbying and campaign contributions reaped some $4.4 trillion in federal business and subsidies for those corporations. They do it because it works.

But it doesn’t work for democracy. We need to revive the traditional understanding of corruption, overturn Citizens United and continue the long American fight for freedom from powerful interests.

What will that mean? First and foremost, it means public financing of elections, which Teddy Roosevelt called for long ago. Public financing allows candidates who show some threshold amount of public support to receive sufficient funding for their campaigns. Congressman John Sarbanes has introduced legislation that would match small contributions with public funds, and free candidates and officeholders from begging from big donors. This legislation would broaden and diversify democratic participation, in our voters and candidates alike.

Only with independence from outside interests can our representatives disentangle from the corruption of our current system.

Explore these other perspectives:

Trevor Burrus: When it comes to politics, corruption is subtler than you think

Jacob Rowbottom: The U.S. and Britain see corruption very differently. Here’s why.

Stephen K. Medvic: The U.S. political system is flawed but not corrupt

Daniel Treisman: Does more wealth mean less political corruption?

Nancy Gertner: The corruption continuum — When giving gifts bleeds to bribery