Since the Supreme Court’s Citizens United decision paved the way for unlimited corporate spending on elections, shadowy political organizations that don’t disclose their donors have spent over $600 million on federal races.

With the Koch brothers’ network alone planning to spend up to $900 million more leading up to the 2016 elections, it’s clear that the scourge of secret spending is only going to get worse. Even Supreme Court Justice Anthony Kennedy, who wrote the Citizens United decision, admitted recently that disclosure in the post-Citizens United world is “not working the way it should.”

Fortunately, President Barack Obama can help bring secret money into the light with just the stroke of a pen. All it would take is an executive order requiring companies that get federal contracts — including at least 70 of the Fortune 100 — to disclose their secret political spending.

It’s already the case, and has been for over seven decades, that federal contractors aren’t allowed to give directly to candidates, political committees, or parties. But in the wake of Citizens United, these companies can make unlimited contributions to outside “dark money” groups that influence our elections from the shadows.

Companies don’t have to tell the public about this kind of spending, and most of them don’t do it voluntarily.

This arrangement opens up all kinds of questions about federal contracting. How much money are our nation’s biggest companies already pouring into our elections without us knowing? Are contracts going to the companies best able to do the job, or to the companies best able to play the political money game?

By giving the public a way to see behind this veil of secrecy, Obama can help voters answer these questions. Why should our taxpayer money go to companies who use it to sway our elections in secret?

Americans deserve a transparent democracy, and Obama has made clear that he agrees. Time and again, he’s spoken out against secret money in politics, calling it “a threat to our democracy” and saying that it’s “time to reverse this trend” of “dark money flood[ing] our airwaves.”

But he’s yet to act on it. The space between Obama’s words and actions will define his legacy on money in politics, and time is running out.

Will he be remembered as a president who boldly took action to shine a light on secret money? Or as a president who missed a critical opportunity to fix it?