Ellen L. Weintraub is a member of the Federal Election Commission.

The White House counsel operates out of the public eye but has the president’s ear. In the Trump administration, with the unprecedented multiplicity of conflict-of-interest challenges facing the businessman-president, the job will take on added importance. As a Democratic commissioner at the Federal Election Commission, I served five years alongside Donald F. McGahn, President-elect Donald Trump’s choice for the post. My experience may be instructive — and disquieting.

The FEC’s fundamental mission is to fight corruption by shining a light on money in politics, empowering citizens to assess their elected officials’ potential conflicts of interest. From the moment he walked in the door in 2008, McGahn made no secret of his disdain for the agency, its mission and the commission staff.

At the six-member FEC, McGahn corralled his two fellow Republicans into a rigid voting bloc, promoting gridlock and delay. In decision after decision, he ensured that the money flooding our political system grew ever murkier and the connections between donors and candidates harder to trace.

Donald F. McGahn in Trump Tower in New York on Nov. 15. (Drew Angerer/Getty Images)

[If Republicans really want to drain the swamp, here’s how to do it]

Appointed to be an arbiter of campaign-finance complaints, McGahn instead assumed the mantle of defense counsel, making an art form of devising byzantine arguments against investigating alleged wrongdoing. Should we investigate groups that spend millions on political ads without disclosing a single donor? Not as he saw it. When a wealthy donor chartered a plane to bring hundreds of volunteers to a candidate’s fundraising event, could that possibly have been an in-kind contribution to the candidate? Again, no. When making its decisions, must the commission blind itself to public information known by any reader of The Post? Amazingly, according to McGahn, yes.

Agency dysfunction was not a byproduct of McGahn’s approach — it was the goal. In the wake of the Supreme Court’s 2010 Citizens United decision, McGahn vetoed proposed rules aimed at ensuring disclosure of the sources of political spending, barring employees from being coerced to support their bosses’ political choices, keeping foreign interests from influencing our elections and addressing the new political powerhouses known as super PACs.

I have served on the FEC for 14 years, with 14 commissioners. While disagreements are nothing new at the FEC, commissioners on both sides of the aisle used to understand that serving on a commission composed of three Democratic and three Republican appointees required compromise and that it was our job to make the agency work. No other commissioner has been as intransigent, as hostile to other points of view and as determined to undermine the law and the commission as McGahn was. The example he set hampers the agency to this day.

Was McGahn motivated by libertarian fervor or by the sense that his partisans would take more aggressive advantage of the rule-free zone he worked to create? I don’t pretend to have a window into his motives. But the results are inarguable: During the five fiscal years before McGahn arrived (2003-2007), the commission negotiated more than $16 million in civil penalties in major cases. Even as the cost of campaigns skyrocketed over the next five years, from 2009 to 2013, McGahn’s relentless obstruction caused penalties to plummet 79 percent, to just $3.3 million. By his final year, we could muster the votes to launch only four investigations.

The White House counsel interprets a great many separation-of-powers issues, and here again McGahn’s tenure at the FEC gives pause. It is the role of commissioners to enforce laws adopted by Congress, unless and until they are struck down by the courts. But McGahn substituted his own view of the Constitution. As he once boasted, “I’m not enforcing the law as Congress passed it. I plead guilty as charged.”

And it’s not just Congress McGahn ignored. Even though the Supreme Court resoundingly endorsed the benefits of disclosure in Citizens United, McGahn blocked all attempts to rein in unreported dark-money spending.

The Post’s Matea Gold looks at how President-elect Donald Trump’s campaign pledge to “drain the swamp” of special interests and big donors compares with his Cabinet picks so far. (Bastien Inzaurralde/The Washington Post)

[The Trump Cabinet: Populists or plutocrats?]

How will McGahn view his new role? Service as White House counsel is not a quest to unearth clever legal loopholes. It requires viewing public office as a public trust, where officeholders aspire to the highest ethical standards, regardless of what they think they might get away with. With Congress unlikely to exercise robust oversight, it falls all the more to the White House counsel to ensure that the administration acts ethically and honorably.

The early signs are not promising. The president-elect has already asserted that “the president can’t have a conflict of interest.” His inaugural team is peddling special access for million-dollar donors.

During his campaign, the president-elect characterized our political system as corrupt and vowed to “drain the swamp.” The last time McGahn’s job was to regulate corruption, he instead gleefully paralyzed the agency charged with enforcing the law.

So what are the odds that a White House counseled by him will take any credible steps to drain the ethical swamp? My experience with McGahn does not offer much basis for optimism.