IN ANNOUNCING an end to the program providing temporary relief from deportation to young immigrants brought into the country illegally as children, both President Trump and Attorney General Jeff Sessions have been clear: The responsibility of saving the "dreamers" is now on Congress's shoulders. House Speaker Paul D. Ryan (R-Wis.) has sent encouraging signals so far, promising to push forward with legislation and telling dreamers to "rest easy." That's a praiseworthy response, though the devil will undoubtedly lie in the details of whatever compromise Mr. Ryan works to hammer out.

In the meantime, Mr. Trump and Mr. Sessions owe the public a better explanation as to why the president rescinded the Deferred Action for Childhood Arrivals (DACA) program, which allowed young undocumented immigrants to work and live freely in the United States through renewable two-year deferrals of deportation. The administration's vague and haphazard account of the legal reasoning behind DACA's termination is far from a sufficient rationale for a decision on which so many lives rely.

Mr. Sessions argued Tuesday that the Trump administration was forced to end DACA after the Justice Department found the program unconstitutional. Yet what did the attorney general mean by that? His public statements provide few details beyond pointing to general concerns about presidential overreach into an area of congressional authority and citing a court ruling against President Barack Obama's would-be extension of DACA to parents of U.S. citizens. Which actions does the Justice Department now believe Mr. Trump is constitutionally prohibited from taking?

Likewise, if Mr. Sessions considers DACA to be unlawful, why is the administration allowing six months for the policy to wind down instead of halting it immediately? There may well be serious, prudential reasons behind this choice: An immediate rescission would have cast 800,000 dreamers suddenly adrift. Yet after finding the policy unconstitutional, Mr. Sessions should explain why the six-month delay is more than an effort by Mr. Trump to seek political cover for an unpopular decision.

Hours after Mr. Sessions's announcement, Mr. Trump tweeted that he would "revisit" DACA if Congress were unable to "legalize" the policy by March. It is unclear just what the president intends with this pledge. If he plans to reinstate or extend DACA, on what basis will he do so, given his own attorney general's determination that he lacks the necessary legal authority?

These questions may seem abstract when compared with the nuts and bolts of immigration legislation or the sickening uncertainty now faced by the dreamers. Yet the answers go to the heart of why these young people face uncertainty and what remedies Mr. Trump would have the power to offer should Congress fail to find a solution. Of course, it would be a mistake to hang too much on the words of this most mercurial of presidents. But having offered young immigrants a possible lifeline, the president has a responsibility to explain to them whether he is legally capable of following through with it — and why he snatched it away from them in the first place.