Likelihood of success on the merits is the standard a court uses to decide whether your underlying legal claim is strong enough to justify a preliminary injunction or temporary restraining order, and it generally means showing you are more likely than not to win at trial. The Supreme Court treats this kind of early intervention as an extraordinary remedy requiring a clear showing, so the merits question functions as a gatekeeper. If your legal claim looks weak, the court stops there and denies the motion. Meeting the burden is less about rhetoric and more about assembling a legal theory the court can rely on and an evidentiary record that supports it under time pressure.
What “Likely to Succeed” Actually Means
The word “likely” does real work here. Most legal commentators read it to mean “more likely than not,” which translates to at least a fifty-one percent probability of prevailing at trial. 1Vanderbilt Law Review. Hazy Shades of Winter: Resolving the Circuit Split over Preliminary Injunctions You are not proving your entire case at this stage. You are making a preliminary showing on the evidence available, and the judge is making a predictive judgment about where the case will land after full discovery and trial.
This is a much heavier lift than the earlier procedural hurdles you may be used to. Surviving a motion to dismiss requires only plausible allegations on paper. Surviving summary judgment requires showing a genuine dispute of material fact. A preliminary injunction demands more than either: the court must conclude, after reviewing actual evidence, that your legal position is the one that will probably win. Judges are being asked to exercise extraordinary power before trial, and they want to see a case that justifies it.
The judge evaluates both the legal theory and its factual support. On the legal side, the question is whether established statutes, regulations, or precedent support the relief you seek. On the factual side, the question is whether the evidence you have presented makes your version of events credible. A strong legal theory collapses without a factual record behind it, and a compelling set of facts means nothing if no law gives you a remedy.
Where the Merits Prong Fits in the Four-Factor Test
Likelihood of success does not stand alone. It is one piece of the four-part test the Supreme Court formalized in Winter v. Natural Resources Defense Council, Inc. (2008). To obtain a preliminary injunction in federal court, you must establish:
- Likelihood of success on the merits
- Likely irreparable harm without the injunction
- A balance of equities tipping in your favor
- That an injunction serves the public interest
The Court emphasized that a preliminary injunction is “an extraordinary remedy never awarded as of right” and that irreparable injury must be “likely,” not merely possible. 2Justia. Winter v. Natural Resources Defense Council, Inc. Rule 65 of the Federal Rules of Civil Procedure governs the procedural mechanics for both preliminary injunctions and TROs, but the substantive four-factor test comes from case law. 3Legal Information Institute. Federal Rules of Civil Procedure Rule 65 – Injunctions and Restraining Orders A weak showing on the merits usually ends the inquiry regardless of how strong the other three factors look.
The Standard Varies by Circuit
Before Winter, many federal courts applied a sliding scale: an extremely strong showing of irreparable harm could compensate for a somewhat weaker showing on the merits. The best-known version is the Second Circuit’s “serious questions” test, which allowed relief where the movant raised questions “so serious, substantial, difficult and doubtful, as to make them a fair ground for litigation and thus for more deliberate investigation.” 1Vanderbilt Law Review. Hazy Shades of Winter: Resolving the Circuit Split over Preliminary Injunctions That is a meaningfully lower bar than “more likely than not.”
After Winter, the circuits split on whether this flexibility survived. The Fourth and Tenth Circuits concluded that Winter eliminated the sliding scale and requires a strict standalone showing of likely success. The Second, Third, Seventh, Ninth, and D.C. Circuits have continued applying their traditional flexible tests. 4Houston Law Review. Preliminary Injunctions in Public Law: The Merits Where you file matters. In a strict circuit, “likely to succeed” means just that. In a flexible circuit, a powerful showing of irreparable harm may let you clear the merits bar with something closer to a serious legal question than a probable victory.
Mandatory Injunctions Require a Stronger Showing
The type of injunction you request changes how demanding the merits showing becomes. A prohibitory injunction asks the court to stop someone from doing something, preserving the current state of affairs until trial. A mandatory injunction asks the court to force the other side to take affirmative action. Courts treat mandatory injunctions as more extraordinary than prohibitory ones, and the burden on the moving party is correspondingly higher. 4Houston Law Review. Preliminary Injunctions in Public Law: The Merits
The practical difference matters. Asking a court to block a competitor from launching a product until your patent case resolves is prohibitory, and the standard four-factor analysis applies. Asking that same court to order the competitor to recall products already on shelves and issue corrective advertising is mandatory relief, and the judge will demand a stronger merits showing before granting it. Going in with the wrong expectation about which type you need is a fast way to lose the motion.
Evidence That Actually Meets the Burden
Preliminary injunction hearings often happen on compressed timelines, sometimes within days or weeks of filing. That reality shapes what evidence you can realistically assemble and how courts evaluate it.
The foundation of most motions is the sworn affidavit or verified complaint. Under Rule 65, a TRO can issue based on “specific facts in an affidavit or a verified complaint” showing immediate and irreparable injury. 3Legal Information Institute. Federal Rules of Civil Procedure Rule 65 – Injunctions and Restraining Orders These documents carry the weight of testimony under penalty of perjury, which gives the judge a reliable starting point. Expert declarations fill in where the facts require specialized knowledge: a damages expert quantifying financial harm, a technical expert explaining trade secret misappropriation, or a scientist describing irreversible environmental damage.
Documentary evidence does heavy lifting. Contracts showing the terms that were breached. Emails capturing the defendant’s knowledge or intent. Financial records demonstrating the scale of the harm. If depositions have already occurred, transcripts containing direct admissions from the opposing party are especially powerful because they preempt credibility disputes.
One nuance catches people off guard: courts apply relaxed evidentiary rules at the preliminary injunction stage compared to trial. Hearsay and other evidence that would be excluded at trial may be considered, with the judge treating admissibility concerns as going to weight rather than exclusion. This flexibility exists because the proceedings are inherently informal and emergency relief would be impossible under full trial-quality rules. That said, admissible evidence is still more persuasive than inadmissible evidence. Relying exclusively on hearsay when firsthand declarations are available is a mistake.
When you need evidence the other side holds, courts have broad discretion to grant expedited discovery specifically to support a preliminary injunction motion. The typical standard is that some unusual circumstance would prejudice you if you had to wait for the normal discovery timeline. Judges are receptive to these requests because better evidence leads to better preliminary decisions.
What Works Against You
Your ability to show likely success depends heavily on whether the underlying law is settled. A clear breach of an unambiguous contract term or a straightforward violation of a well-established regulation gives the judge a direct path to a confident preliminary conclusion. Cases pushing novel legal theories or arguing for expansion of existing law face a steeper climb. Judges are reluctant to exercise extraordinary power based on an untested argument that might not survive appellate review.
Factual clarity matters just as much. When the key facts are undisputed or well-documented, the judge can apply the law with confidence. When the record is filled with conflicting testimony and the two sides tell fundamentally different stories, the court often concludes that the merits are too uncertain to warrant immediate intervention. This is where preliminary injunction motions frequently die. Not because the legal theory is wrong, but because the factual record is too muddled for a judge to say the moving party will probably win. A clean factual record built on contemporaneous documents rather than after-the-fact characterizations gives you a meaningful advantage.
There is also an asymmetry worth understanding. A denial of your motion does not mean you lose the case; litigation continues and you get your full day at trial. But if the court grants the injunction and you ultimately lose at trial, the defendant may have been wrongfully restrained for months or years. Judges are acutely aware of this, and it makes them cautious. The stronger and more concrete your evidence is at the preliminary stage, the more comfortable the court will be acting before the full picture emerges.
The Merits Showing in TROs
The merits standard applies to both TROs and preliminary injunctions, but the context differs. A TRO is designed for true emergencies. Under Rule 65(b), a court can issue one without any notice to the opposing party if you demonstrate through an affidavit or verified complaint that immediate and irreparable injury will result before the other side can be heard, and your attorney certifies in writing what efforts were made to give notice and why notice should not be required. 3Legal Information Institute. Federal Rules of Civil Procedure Rule 65 – Injunctions and Restraining Orders Because TROs are issued under extreme time pressure and sometimes without the other side present, judges scrutinize the factual showing in the supporting affidavits with particular care. Specificity in the sworn statements is what carries the merits argument when no adversarial testing has occurred yet.
Appellate Review of the Merits Finding
Unlike most pretrial rulings, a decision to grant or deny a preliminary injunction is immediately appealable. Under 28 U.S.C. ยง 1292(a)(1), federal appellate courts have jurisdiction to review interlocutory orders granting, denying, modifying, or dissolving injunctions without waiting for final judgment. 5Office of the Law Revision Counsel. 28 U.S. Code 1292 – Interlocutory Decisions
Appellate courts review the district court’s decision for abuse of discretion, meaning they will not overturn a ruling simply because they would have weighed the factors differently. Errors of law made during the analysis, such as applying the wrong standard to the merits prong, are reviewed without deference. If the district court misidentified the governing standard or failed to consider one of the four Winter factors, that legal error can support reversal even when the factual findings were reasonable. The practical takeaway for anyone appealing: challenge the judge’s legal framework, not just the outcome.