ON JANUARY 19th, almost a year after Donald Trump issued his initial ban on travel from Muslim countries, the Supreme Court agreed to resolve a challenge to a third, more refined version of that order. The case, Trump v. Hawaii, arises from the Ninth Circuit Court of Appeals, an unfriendly audience for several of Mr Trump’s travel restrictions. (A ruling from the Fourth Circuit Court of Appeals in Trump v International Refugee Assistance Project, a similar challenge, is still pending.) The justices have managed to stay mostly on the sidelines of the legal wrangling over Mr Trump’s campaign promise to protect America from terrorism by roping out certain foreigners, but the highest court in the land will now weigh in directly. 

The latest version of the travel ban is a few steps removed from the president's campaign-trail call in December 2015 for “a total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what is going on”. The first, hastily drafted executive order on January 27th 2017 wreaked havoc as travellers flying to America from Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen—green-card holders and foreigners with visas—were turned away upon arrival. The move was quickly blocked by federal judges, and the White House backed off, pivoting to a somewhat softened order in March that exempted lawful permanent residents, struck Iraq from the banned list, and removed language suggesting that Christians from the affected countries would enjoy special treatment. 

But the second ban fared no better in the eyes of federal judges, drawing injunctions from district and appellate courts on both coasts. In late June, in response to requests for emergency stays on these rulings from the White House, the Supreme Court came up with a short-term compromise. The travel ban could be implemented for the time being, the justices said, but not against foreigners with ties to people living in America. Just before the justices’ planned hearing date to fully consider the issue in October 2017, the Trump administration withdrew the order and replaced it with a third.

Travel ban 3.0 tweaks the details and reshuffles the list of banned countries (removing Sudan and adding Chad, North Korea and Venezuela). On October 17th, the same federal judge in Hawaii who blocked the second ban wrote a decision freezing the third. It “suffers from precisely the same maladies as its predecessor”, he wrote, despite its window-dressing addition of a pair of non-Muslim countries. A Maryland judge issued an injunction against the third ban too, just hours later. But the Supreme Court (over the dissenting votes of only Ruth Bader Ginsburg and Sonia Sotomayor) stayed those rulings in early December while litigation pressed on. 

What is the legal basis for challenging the president’s order, which opponents continue to deride as a “Muslim ban”? There are two tacks: constitutional and statutory, and the Supreme Court has asked the parties to address both in their briefs. The constitutional worry focuses on the First Amendment’s ban on policies “respecting an establishment of religion”; discriminating against Muslims in immigration policy may disparage an entire religion and reflect official government hostility toward Muslims both abroad and at home. There is ample evidence from the campaign trail and from Mr Trump’s famously intemperate tweets and public statements as president that suspicion of Muslims fuels his travel restrictions. It’s conceivable that Justice Anthony Kennedy—for whom the protection of human dignity is among the constitution's most important values—may join liberal justices in signing on to this reading of the First Amendment. But the Supreme Court has never held that a president’s pre-inaugural statements or social-media posts taint a policy with an unconstitutional suggestion of animus. It seems unlikely that a majority of the justices would go for such a line of reasoning.

More promising for the travel ban’s challengers is the argument that Mr Trump lacked authority under federal immigration laws to issue such sweeping restrictions. This is the basis on which the Ninth Circuit found against Mr Trump on December 22nd. The appeals court in California examined the Immigration and Naturalisation Act and found that only Congress is equipped to implement an indefinite ban on issuing visas to blocs of foreigners. While the president may temporarily block particular groups of people from accessing America’s shores, the three-judge panel wrote, a president “cannot without the assent of Congress supplant its statutory scheme with one stroke of a presidential pen”. Mr Trump may seek to “prevent entry of terrorists and persons posing a threat to public safety”, but “Congress has already acted to effectuate these purposes”. 

When the Supreme Court hears Trump v Hawaii—probably in late April, with a decision coming by the end of June—the Ninth Circuit’s less ideologically fraught approach may prick the ears of justices who worry about presidents becoming too big for their breeches. Citing a decision by a conservative federal judge in Texas who blocked Barack Obama’s order protecting undocumented immigrants in 2015, the Ninth Circuit judges noted that when there are “serious concerns that the president has not” acted within the scope of the law, the public interest is best served by “curtailing unlawful executive action.” They also made a broader case for rejecting Mr Trump’s restrictions. It can’t be in public interest, they wrote, “that a portion of this country be made to live in fear”.