From Citigroup Global Markets, Inc. v. VCG Spec. Opportunities Master Fund Ltd., 598 F.3d 30 (2d Cir. 2010):

VCG argues that three recent decisions of the Supreme Court—Munaf v. Geren, 553 U.S. 674, 128 S. Ct. 2207, 171 L. Ed. 2d 1 (2008); Winter [v. Natural Resources Defense Council, Inc. , 129 S. Ct. 365, 172 L. Ed. 2d 249 (2008); and Nken v. Holder, 129 S. Ct. 1749, 173 L. Ed. 2d 550 (2009)—have eliminated this circuit's "serious questions" standard for the entry of a preliminary injunction, and that, in light of the district court's finding that CGMI failed to demonstrate its likelihood of success on the merits, the entry of a preliminary injunction in this case must be reversed. ***

Winter articulates the following standard for issuing a preliminary injunction:

A plaintiff seeking a preliminary injunction must establish that he is likely to succeed on the merits, that he is likely to suffer irreparable harm in the absence of preliminary relief, that the balance of equities tips in his favor, and that an injunction is in the public interest.

Winter, 129 S. Ct. at 374; see also Munaf, 128 S. Ct. at 2219; Nken, 129 S. Ct. at 1761. Although not stated explicitly in its briefs, we take VCG's position to be that the standard articulated by these three Supreme Court cases requires a preliminary injunction movant to demonstrate that it is more likely than not to succeed on its underlying claims, or in other words, that a movant must show a greater than fifty percent probability of success on the merits. Thus, according to VCG, a showing of "serious questions" that are a fair ground for litigation will not suffice. ***

I. The Continued Viability of the "Serious Questions" Standard

For the last five decades, this circuit has required a party seeking a preliminary injunction to show "(a) irreparable harm and (b) either (1) likelihood of success on the merits or (2) sufficiently serious questions going to the merits to make them a fair ground for litigation and a balance of hardships tipping decidedly toward the party requesting the preliminary relief." *** The "serious questions" standard permits a district court to grant a preliminary injunction in situations where it cannot determine with certainty that the moving party is more likely than not to prevail on the merits of the underlying claims, but where the costs outweigh the benefits of not granting the injunction. *** Because the moving party must not only show that there are "serious questions" going to the merits, but must additionally establish that "the balance of hardships tips decidedly" in its favor, *** its overall burden is no lighter than the one it bears under the "likelihood of success" standard.

[Footnote 4] We have recognized three limited exceptions to this general standard, none of which is relevant here. First,

[W]here the moving party seeks to stay government action taken in the public interest pursuant to a statutory or regulatory scheme, the district court should not apply the less rigorous ["serious questions"] standard and should not grant the injunction unless the moving party establishes, along with irreparable injury, a likelihood that he will succeed on the merits of his claim.

Able v. United States, 44 F.3d 128, 131 (2d Cir. 1995).....

Second, "[a] heightened 'substantial likelihood' standard may also be required when the requested injunction (1) would provide the plaintiff with 'all the relief that is sought' and (2) could not be undone by a judgment favorable to defendants on the merits at trial." Mastrovincenzo v. City of New York, 435 F.3d 78, 90 (2d Cir. 2006)***.

Third, a "mandatory" preliminary injunction that "alter[s] the status quo by commanding some positive act," as opposed to a "prohibitory" injunction seeking only to maintain the status quo, "should issue 'only upon a clear showing that the moving party is entitled to the relief requested, or where extreme or very serious damage will result from a denial of preliminary relief.'" Tom Doherty Assocs., 60 F.3d at 34 ....

The value of this circuit's approach to assessing the merits of a claim at the preliminary injunction stage lies in its flexibility in the face of varying factual scenarios and the greater uncertainties inherent at the outset of particularly complex litigation. Preliminary injunctions should not be mechanically confined to cases that are simple or easy. ***

[Footnote 5] We note that, prior to Winter, seven of the twelve regional Courts of Appeals*** applied a preliminary injunction standard that permitted flexibility when confronting some probability of success on the merits that falls short of a strict fifty-one percent. [Citations omitted.]

Indeed, the Supreme Court, prior to the trilogy of cases cited by VCG, has counseled in favor of a preliminary injunction standard that permits the entry of an injunction in cases where a factual dispute renders a fully reliable assessment of the merits impossible. In Ohio Oil Co. v. Conway, 279 U.S. 813, 49 S. Ct. 256, 73 L. Ed. 972 (1929), the Court dealt with a factual dispute, relating to the effect on the plaintiff of a state tax on oil revenues, which had to "be resolved before the constitutional validity of [a] statute [could] be determined." *** Faced with this situation, the Court instructed that "[w]here the questions presented by an application for an interlocutory injunction are grave, and the injury to the moving party [in the absence of such an injunction] will be certain and irreparable . . . the injunction usually will be granted."***

The Supreme Court's recent opinions in Munaf, Winter, and Nken have not undermined its approval of the more flexible approach signaled in Ohio Oil. None of the three cases comments at all, much less negatively, upon the application of a preliminary injunction standard that softens a strict "likelihood" requirement in cases that warrant it. Munaf involved a preliminary injunction barring the transfer to Iraqi custody of an individual captured in Iraq by the Multinational Force-Iraq. Munaf, 128 S. Ct. at 2214-15. That injunction was premised on "jurisdictional issues . . . so serious, substantial, difficult and doubtful, as to make them fair ground for litigation and thus for more deliberative investigation." *** The Supreme Court vacated that injunction on the grounds that a "likelihood of jurisdiction" was irrelevant to the preliminary injunction consideration and could not substitute for a consideration of the merits. The Court in Munaf simply stated that a question as to a court's jurisdiction over a claim "says nothing about the 'likelihood of success on the merits,'" *** but provided nothing in the way of a definition of the phrase "a likelihood of success." ***

Nor does Winter address the requisite probability of success of the movant's underlying claims. While Winter rejected the Ninth Circuit's conceptually separate "possibility of irreparable harm" standard, 129 S. Ct. at 375-76, it expressly withheld any consideration of the merits of the parties' underlying claims, id. at 376, 381. Rather, the Court decided the case upon the balance of the equities and the public interest. 129 S. Ct. at 375-76, 381.

Finally, Nken likewise did not address the issue of a moving party's likelihood of success on the merits. Nken provides a four factor standard for granting a stay pending appeal, which the Court recognized as overlapping substantially with the preliminary injunction standard. 129 S. Ct. at 1761. Although the Court repeated the "likely to succeed on the merits" phrasing, it did not suggest that this factor requires a showing that the movant is "more likely than not" to succeed on the merits.

If the Supreme Court had meant for Munaf, Winter, or Nken to abrogate the more flexible standard for a preliminary injunction, one would expect some reference to the considerable history of the flexible standards applied in this circuit, seven of our sister circuits, and in the Supreme Court itself. We have recognized this flexible standard since at least 1953.... We have found no command from the Supreme Court that would foreclose the application of our established "serious questions" standard as a means of assessing a movant's likelihood of success on the merits. Our standard accommodates the needs of the district courts in confronting motions for preliminary injunctions in factual situations that vary widely in difficulty and complexity. Thus, we hold that our venerable standard for assessing a movant's probability of success on the merits remains valid and that the district court did not err in applying the "serious questions" standard to CGMI's motion.