The 2016 campaign is just beginning, but we already know that hundreds of millions of secret dollars will be spent over the next 18 months. A few years ago, this tide of “dark money” would have been unimaginable.

Today, it represents one of the biggest threats to our democracy. President Obama has spoken out against the rise of dark money, but has done little else to combat it. With a hostile Supreme Court and Congress, many have assumed this is the best he can do – but that’s just not true.

In fact, the president has the power to strike a major blow against dark money in our elections now, without congressional approval, and without running afoul of Supreme Court precedent. He can issue an executive order to expose secret political spending by federal contractors. The only question is whether he will follow through.

Since the Supreme Court’s Citizens United decision in 2010, political spending by groups that conceal their donations from the public has skyrocketed, to the tune of well over $600 million. As a result, voters increasingly don’t know who is behind political ads, or what those advertisers really want from the government.

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This is a big problem. As Chief Justice John Roberts, no friend of most campaign finance laws, pointed out in last year’s McCutcheon decision, disclosure helps to prevent “abuse of the campaign finance system.” Support for a candidate from a local veteran’s group, for instance, is very different than support from a giant defense contractor. Keeping voters in the dark about who is really trying to sway them eliminates one of the last remaining checks on corruption of our government by special interests.

While no executive order can fix this problem entirely, the action we urge will bring to light secret spending that is especially troubling: dark money from federal contractors seeking taxpayer dollars. As former SEC Chair Mary Schapiro said back in 2010, giving public officials gifts or contributions to influence contracting decisions (known as “pay to play”) is “an unspoken, but entrenched and well understood practice.” The federal government spent almost $460 billion in FY2013 on private sector contracts, almost 40% of which went to just 25 companies. Since 2000, the top 10 federal contractors have made $1.5 trillion from the government.

With this much money at stake, the benefits of courting those in power are obvious. In the 2014 election cycle alone, the top 25 federal contractors gave more than $30 million in disclosed contributions through their corporate PACs. There is nothing to prevent these same companies from giving untold millions to dark money groups. And since such groups increasingly back only one candidate, giving to them allows donors to target a particular race exactly as they would do with ordinary contributions — only without limits and in secret.

The politicians who benefit from such largesse will naturally feel gratitude towards their benefactors. When that benefactor is a federal contractor, opportunities to reciprocate its generosity by steering federal money in its direction are legion.

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The result is a system in which contracts go to those best able to play the political money game, rather than those offering the best, most cost-effective product or service – with taxpayers footing the bill. Worse, since the federal government relies on private contractors to, among other things, supply our military, care for our veterans, guard our embassies, manage our prisons, and test the quality of our air and water, pay-to-play practices can have a huge impact on American lives.

An executive order is the only way to fix this problem now. Congress had the chance to address dark money by passing the DISCLOSE Act, but in 2010 the legislation was filibustered in the Senate, fell one vote short of the 60 needed to end debate, and has sat on the shelf ever since. The FEC, our nation’s beleaguered campaign finance regulator, remains perpetually mired in gridlock. And while there is more hope for action on political disclosure from the SEC and the IRS, neither agency is likely to act before the 2016 election.

We have been down this road before. In 2011, the Obama administration appeared poised to issue an executive order to require prospective contractors to disclose political spending as part of submitting a bid, but the proposal was shelved in the face of criticism that it would inject politics into the bid process. If that is a real concern, though, there is an easy fix. Simply require disclosure only from the winning bidder, and only after the contract has been awarded.

By issuing an executive order, Obama can take a concrete step to promote transparency and leave a real, substantive legacy on this issue. Or he can do nothing – and watch the dark money explosion become our new normal. Lawrence Norden and Daniel Weiner

President Obama cannot afford to delay. The 2016 race for the White House is already underway, and any order he issues will require time to implement. It won’t address every threat to the integrity of our political system, or even address the entirety of the dark money problem. But it is the biggest thing that can be done today.

The president has made the quest for a “better politics” a rhetorical hallmark of his career. Now he has a chance to put his own words into action. By issuing an executive order, he can take a concrete step to promote transparency and leave a real, substantive legacy on this issue. Or he can do nothing – and watch the dark money explosion become our new normal. Mr. President, which will it be?

Lawrence Norden is deputy director of the Democracy Program at the Brennan Center for Justice at NYU School of Law. Daniel I. Weiner is counsel in the Democracy Program.