All emergency applications are filed in the first instance with the “Circuit Justice” assigned to that particular court of appeals/geographic area. For the Boston-based First Circuit, that’s Justice Jackson. And with one equivocal exception, every “administrative” stay of which I’m aware has come from the Circuit Justice, not the full Court. Thus, the onus was on Justice Jackson to either enter the administrative stay herself, or risk being overruled by the full Court.
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As for why Justice Jackson did it, to me, the clue is the last sentence. Had Jackson refused to issue an administrative stay, it’s entirely possible (indeed, she may already have known) that a majority of her colleagues were ready to do it themselves. I still think that this is what happened back in April when the full Court intervened shortly before 1 a.m., without explaining why Justice Alito hadn’t, in the A.A.R.P. Alien Enemies Act case. And from Jackson’s perspective, an administrative stay from the full Court would’ve been worse—almost certainly because it would have been open-ended (that is, it would not have had a deadline). The upshot would’ve been that Judge McConnell’s order could’ve remained frozen indefinitely while the full Court took its time. Yesterday’s grant of a stay in Trump v. Orr, for instance, came 48 days after the Justice Department first sought emergency relief.

Instead, by keeping the case for herself and granting the same relief, in contrast, Justice Jackson was able to directly influence the timing in both the First Circuit and the Supreme Court, at least for now. She nudged the First Circuit (which I expect to rule by the end of the weekend, Monday at the latest); and, assuming that court rules against the Trump administration, she also tied her colleagues’ hands—by having her administrative stay expire 48 hours after the First Circuit rules. Of course, the full Court can extend the administrative stay (and Jackson can do it herself). But this way, at least, she’s putting pressure on everyone—the First Circuit and the full Court—to move very quickly in deciding whether or not Judge McConnell’s orders should be allowed to go into effect. From where I’m sitting, that’s why Justice Jackson, the most vocal critic among the justices of the Court’s behavior in Trump-related emergency applications, ruled herself here—rather than allowing the full Court to overrule her. It drastically increases the odds of the full Supreme Court resolving this issue by the end of next week—one way or the other.