It'll be a high profile legal assist in what's become one of the most unusual and polarizing criminal matters in DC federal court in years.
Flynn challenged Sullivan's plans last week, going directly to the appeals court about the trial-level judge. That's when an appellate panel gave the surprising order for the judge to explain to them by June 1 his stance and his power over the case.
The small firm, Wilkinson Walsh, will be writing a legal argument on Sullivan's behalf after the appeals court last week gave him the unusual order to defend his authority, the person said.
The three-judge appellate panel includes two Republican-appointed judges who've been sympathetic to the Trump administration regarding cases related to the Mueller investigation and they may be inclined to push back against Sullivan as Republican criticism of the judge has grown.
Wilkinson is one of the most prominent litigators in Washington. She rose to prominence prosecuting Oklahoma City bomber Timothy McVeigh under the supervision of later-Circuit Judge Merrick Garland.
More recently, Wilkinson's firm represented now-Supreme Court Justice Brett Kavanaugh for his confirmation hearings.
Wilkinson is married to CNN contributor David Gregory.