This commentary was originally featured in The Hill on July 29, 2017.

In the opening remarks at his confirmation hearings, Chief Justice John Roberts stated that “judges are like umpires.” But what happens when the umpire decides that balls look like strikes when one team is at bat, and not the other?

Under our federal system, Congress makes laws that authorize executive agencies to carry out certain functions. They can do so using narrow language that prescribes the methods for executing their aspirations, or they can leave the methodology up to the authorized agency. Under the Chevron doctrine, named after the holding in a landmark Supreme Court case, courts should defer to the agency’s interpretation of ambiguous statutory language.

Courts often follow a similar practice when deciding conflicts of state and federal law. In Arizona v. United States, the Supreme Court ruled that the federal government had effectively “occupied the field” of immigration regulation, applying federal policies rather than the stricter statute passed by the state of Arizona.

Chevron and Arizona both express a larger guiding principle of judicial deference to the legislating and policymaking branches of government. Returning to his metaphor, Chief Justice Roberts emphasized that umpires “don’t make the rules… they apply them.”

In theory, and often in practice, judicial deference restrains judicial activism by providing a consistent way to resolve disputes over statutory construction and the reach of federal law. It also incentivizes clear legislative drafting, so if Congress wants a law executed in a certain way, it had better say so.

But judicial deference cannot serve either of these ends if federal judges only apply it when one political party controls the White House, and not the other. Yet there is evidence that left-leaning judges frequently do just that.

Researchers recently concluded that “Chevron deference appears to constrain conservative judges as to liberal [administrative] interpretations, but not vice versa.” When confronted with liberal agency interpretations of ambiguous legal language, all judicial panels — liberal and conservative alike — were equally likely to apply the Chevron doctrine. But when judges confronted conservative agency interpretations, liberal judges applied Chevron deference to such interpretations far less frequently than conservative judges.

There is some indication that this trend is continuing during President Trump’s tenure. The Environmental Protection Agency decided to delay a regulation targeting methane emissions based on a Clean Air Actprovision that allowed such a delay in order to reconsider regulations. Yet the D.C. Court of Appeals decided not to defer to the EPA’s interpretation of this provision, applying its own judgment to determine that the regulation had been considered long enough. The court summarily vacated the Trump EPA’s decision, refusing to apply Chevron.

In the Constitution of the Commonwealth of Massachusetts, passed in 1780, John Adams wrote that the judiciary should not exercise legislative or executive power. This safeguard was necessary, he said, in order to create “a government of laws and not of men.” Judges that see fit to reject the current rule of deference in favor of judicial activism blatantly ignore this wise precept.

Treating judicial deference as a principle that only applies when a judge decrees that it does turns legal baseball into legal Calvinball. If courts decide that the rules don’t apply when precedent leads to a decision they ideologically differ with, the outcome impacts more than the specific case — it undermines the public’s faith and reliance in judicial objectivity.