There’s no basis for the Court to issue such an injunction against the AG, absent a holding by the Court (which Trump is not urging) that enforcement of the Act would be unconstitutional.  Courts do not have a sort of free-floating power to prevent the enforcement of a valid federal statute approved by the two political branches.  Moreover, the Supreme Court has held that when the sovereign is prevented “from effectuating statutes enacted by representatives of its people, it suffers a form of irreparable injury.”  Maryland v. King, 567 U.S. 1301, 1303 (2012) (Roberts, C.J., in chambers).   

To be sure, the All Writs Act (28 U.S.C. 1651) authorizes the Court to issue writs “necessary or appropriate in aid of [its] jurisdiction[] and agreeable to the usages and principles of law.”  Therefore, for example, if the Justices, after oral argument, vote at conference to declare the Act unconstitutional, but they can’t finalize their opinions by January 19, the Court might have the power to enjoin the Attorney General from enforcing the Act until the Court issues its decision.  The injunction Trump is requesting here, however, wouldn’t depend upon even a tentative conclusion by the Court that the Act is unconstitutional.  Such an order therefore wouldn’t be in aid of the Court’s jurisdiction, nor would it be “agreeable to the usages and principles of law.”  [UPDATE (01/04):  In her reply brief, the Solicitor General agrees:  "That requested relief is more properly characterized as a temporary injunction and thus is appropriate only if the plaintiff establishes, among other things, a likelihood of success on the merits.  See Roman Catholic Diocese of Brooklyn v. Cuomo, 592 U.S. 14, 16 (2020) (per curiam); Winter v. NRDC, Inc., 555 U.S. 7, 32 (2008).  Petitioners have not made that showing here—and the President-elect does not argue otherwise."]

Second, the reason Trump offers for the Court to issue such a “stay” is that it would allow him, once he takes office, “to resolve the dispute through political means.”  The only ways for Trump to “resolve the dispute through political means” once he becomes President would be for him either to persuade Congress to amend the Act, or to persuade TikTok and ByteDance to agree to a divestiture of the former from the latter that would satisfy the statutory standards.  Neither of those possibilities, however, offers any reason for the Court to enjoin enforcement of the Act before they might happen.  I hope it’s self-evident that just because Congress might amend a law in the future isn’t a basis for a court to enjoin its enforcement before such an amendment.  And although the Act contemplates that the parties might conclude a qualifying divestiture after January 19, Congress specifically decided that third-party providers must cease supporting TikTok unless and until the President certifies that the parties have finalized that divestiture.  If TikTok were “unsupported” between January 19 and the date of the presidential certification of a divestiture, that would simply be a case of the statute operating as Congress intended—and therefore no injunction would be justified. 

Third—and most ominously in terms of Trump's understanding of his constitutional role in his forthcoming presidentcy—the brief suggests (p.10) that Congress’ decision to apply the Act to TikTok raises a constitutional concern by taking that decision “out of the Executive’s hands.”  This argument appears to be predicated on the mistaken notion (p.15) that “the President alone, not Congress … , is charged with the primary responsibility for the United States’ national security and foreign policy"—and, implicitly, that Congress therefore may not enact statutes interfering with that presidential responsibility.  As the Office of Legal Counsel explained in a recent opinion, the Court in Zivotofsky v. Kerry (2015) unequivocally rejected the notions that the Constitution affords the President “‘the bulk of foreign-affairs powers’” and that the President is “free from the ordinary controls and checks of Congress merely because foreign affairs are at issue.”  “In a world that is ever more compressed and interdependent,” the Court explained, “it is essential the congressional role in foreign affairs be understood and respected.  For it is Congress that makes laws, and in countless ways its laws will and should shape the Nation’s course.”  576 U.S. at 21.

Fourth, Sauer also suggests (p.11) that there’s something constitutionally problematic about a law going into effect the day before a new President takes the oath of office.  That’s absurd.  An incoming President is constitutionally obligated to take care that all valid, existing statutes are faithfully executed, whether they went into effect 100 years before the President assumed office or only minutes before he or she takes the oath—or if they go into effect next July, or if Congress enacts a new law over the President’s veto during the President’s own Term.  Trump's Acting Attorney General therefore will have the same exact responsibility to enforce the Act on January 20 as Merrick Garland will have on January 19, because they will both hold the same office.  There’s nothing unusual or constitutionally suspect about that.         

Fifth, Trump insists in the brief (p.3) that he “opposes banning TikTok in the United States at this juncture.”  He further warns (p.6) that “the First Amendment implications of the federal government’s effective shuttering of a social media platform used by 170 million Americans are sweeping and troubling” and that “the Act may set a dangerous global precedent by exercising the extraordinary power to shut down an entire social-media platform.”  Yet Trump’s brief never even mentions that in 2020, when he was President, Trump himself issued one Executive Order effectively prohibiting persons in the United States, including TikTok Inc., from engaging in transactions with ByteDance Ltd., and another Order requiring ByteDance itself to divest all interests and rights in any property used to support ByteDance’s operation of TikTok in the United States and any data obtained or derived from TikTok’s U.S. users.  Several district courts ruled that the former Order was invalid because President lacked statutory authority under the International Emergency Economic Powers Act (IEEPA) to impose such restrictions, and President Biden later rescinded that Order.  Yet President Trump’s second Order, requiring ByteDance to divest from TikTok Inc., technically remains in effect, though it’s never been enforced.  (TikTok challenged that Order in federal court, see TikTok v. CFIUS, No. 20-1444 (D.C. Cir.), and the court of appeals placed the case “in abeyance” at the parties’ urging.  Once Congress passed the Act and TikTok Inc. challenged it in the case now before the Supreme Court, the parties informed the court of appeals that abeyance “continues to be appropriate to avoid potentially duplicative litigation and to conserve judicial resources.") 

Trump’s brief cites his two Orders (p.15) for the proposition that he has national security “concerns” about TikTok “similar to those that the Government cites to defend the Act.”  He fails to inform the Court, however, that those “concerns” led Trump himself to issue directives that would have had the same effect as the Act—namely, an effective shutdown of TikTok in the U.S.

Sixth, Sauer characterizes the Act (p.5) as “dictat[ing] that the President must make a particular national-security determination as to TikTok.”  That’s simply wrong.  The Act will go into effect as to TikTok on January 19 without any further action by the President.

Seventh, and finally, Sauer writes (p.7) that because the Act “contemplates the possibility of a 90-day extension,” that must mean that the January 19 effective date “lacks talismanic significance.”  Talismans aside, this statement is very misleading.  The Act authorizes the President to grant a single extension, of no more than 90 days, before the Act’s prohibition kicks in, but only if the President certifies to Congress that “a path to executing a qualified divestiture has been identified,” that “evidence of significant progress toward executing” the divestiture “has been produced,” and that “there are in place the relevant binding legal agreements to enable execution of the divestiture during the extension period.”  In other words, the Act includes the presidential “delay” provision not (as the Trump brief implies) because Congress was relatively indifferent as to when steps must be taken to eliminate ByteDance’s (and the PRC’s) effective control over TikTok, but instead only to deal with a situation in which a qualifying divestiture, which would satisfy that objective, is imminent.  (There’s no prospect of the conditions being satisfied before January 19, and therefore President Biden has no power to grant such an extension over the next 17 days.)