Questions about responsibility dominated oral argument on Monday before the US fourth circuit court of appeals, which sat in judgment of Donald Trump’s revised travel ban. By the end of the sitting, it appeared likely that a majority of the court will uphold a ruling against Trump – but that its ruling will draw passionate dissents.

This was no ordinary case. Rather than assign a three-judge panel, the fourth circuit undertook on its own initiative to sit en banc. Through a grueling 90-minute session before the full 13-member court, advocates and judges sparred over standing, statutes and arcane doctrines. Most of the rhetorical firepower, though, was reserved for a single question: whether Trump’s order is based in anti-Muslim animus and thus violates the establishment clause.

Defending the ban, the acting solicitor general, Jeffrey B Wall, relied heavily on a rigid distinction between candidates and presidents. Candidates, he reasoned, cannot be held responsible for what they say; after all, they “talk about things on the campaign trail all the time”. (Apparently, Wall’s definition of “things” covers excluding an entire faith from America.) Trump’s vitriolic anti-Muslim campaign statements are thus totally irrelevant to the analysis. On 19 January 2017, he was a mere private citizen whose publicly stated views on how to use federal power meant nothing at all.

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Presidents, in contrast, have “faced the demands of government”, “consulted with an administration” and sworn an oath of office. They are vested with great power and charged with keeping the nation safe. Presidents therefore enjoy a “presumption of regularity” in their actions and the benefit of every doubt in their statements. They must be trusted to exercise their responsibilities wisely, especially in the realm of immigration and national security.

In Wall’s telling, it’s therefore irresponsible of courts to probe the president’s motives: this risks an order drifting “in and out of constitutionality based on what we think was in the head of the president who issued it”. Instead, courts must confine themselves to the official text of executive orders, ignoring all other context and straining to give the president’s words a favorable gloss.

On this point, Wall was supported by judges Paul V Niemeyer and Dennis Shedd. As they pointed out, it can be virtually impossible to gauge motive, especially when the president has consulted many officials. Further, motive-based tests can lead to anomalous results. For instance, if Hillary Clinton had issued the exact same order, would it be lawful? What if Trump publicly apologized every day for a year, disclaiming animus – would that save the order?

Ultimately, Judge Shedd worried that judicial second-guessing of the president’s motive would blur responsibility for public policy: “Can the president do anything? … And if he can’t do that, and if something happens in that time period, who’s responsible?” As Judge Shedd noted, in the event of an immigrant terrorist attack, the American people will ask why something hadn’t been done to thwart disaster – and the president will say: “I tried to.”

These are reasonable, important concerns. That said, a majority of the court appeared unconvinced, and for good reason.

To start, supreme court precedent requires courts to assess presidential motive here. As I described in a brief on behalf of legal scholars, under the establishment clause, official acts based in animus toward any particular religion are forbidden. And as professor Daniel Hemel has explained, under relevant immigration cases, “a decision to exclude aliens from the country can be struck down upon a convincing showing of bad faith”. Several judges, including Pamela Harris and James Wynn Jr, properly emphasized that these rules compel a judicial analysis of Trump’s purpose.

This raises an important question, posed directly by Judge Wynn: “Which are the statements that are relevant here?” Despite Wall’s effort to shield Trump from responsibility for his many anti-Muslim remarks, the correct answer – as many commentators have explained – is all of them reasonably bearing on this policy.

Any other conclusion would be destructive of our constitutional culture. The notion that the president can speak without any sense of legal responsibility for his statements is chilling. And it would be disturbing for Trump’s most controversial policies to be upheld only because courts deliberately blind themselves to widely shared understandings – based in Trump’s own remarks – of what those policies mean and why they were enacted. In that event, the public might quickly come to think the constitution means awfully little.

It’s true, of course, that the president typically enjoys a judicial presumption of good faith and regularity. But surely there comes a point where reliance on this rule amounts to judicial abdication – and Trump’s continuing bad faith and irregularity suggest we have crossed that Rubicon. Even if we haven’t, the nature of presumptions is that they can be rebutted, and here the evidence of Trump’s bad faith toward American Muslims is overwhelming.

Yes, this means the exact same order could be lawful under one president and unlawful under another. Yes, this means courts must reckon with fact-intensive questions about when an improper motive has been cured. And yes, this means courts may have to engage in tough analysis of presidential purpose.

These objections, while forceful, would be a lot more impressive if addressing these very issues weren’t already the standard fare of judges. The alternative, moreover, is to abandon the field entirely. In this day and age, that option is singularly unacceptable.

Which leads back to Judge Shedd’s question: if courts second-guess the president’s national security policies, how can the public know who is ultimately responsible for its security?

In my view, that question rests on a mistaken premise. There are lots of things the president could do that would make us safer, if he were free to ignore the constitution. But the framers chose a different path. As Justice Kennedy once wrote: “The laws and Constitution are designed to survive, and remain in force, in extraordinary times. Liberty and security can be reconciled; and in our system they are reconciled within the framework of the law.”

The president is responsible for protecting national security – and for doing so while respecting the constitution. It’s up to him to create, announce and implement lawful policies. Judges may not invalidate Trump’s travel ban merely because they disagree with it, but they assuredly can strike it down for violating fundamental rights. Our history offers painful lessons about the potential for tragedy when courts fail to exercise this vital responsibility.

At oral argument, Judge Wynn described the extraordinary record in this case, noting the tight link in time, subject and substance between Trump’s campaign promises and the revised travel ban. He asked: “Have we ever had that kind of situation in history?”

We haven’t. Our past offers few examples of facially valid security policies shot through with illegal motive by a volley of presidential statements. But if the judiciary blinds itself, freeing Trump of all responsibility for his words, policies like this may define our future.