It would be hard to script a better hypothetical: Two sisters, 50-year-old identical twins, both born and raised in Pennsylvania, both suffering from myotonic dystrophy, a degenerative disorder affecting muscle function and mental processing.

Leslie Schaller still lives in Pennsylvania and gets a $755 Supplemental Security Income, or SSI, check each month after she was deemed disabled. With this money, she is able to lead a full and independent life.

But her twin Katrina lives in Guam—she moved there permanently in 2008 to live with their other sister, who cares for her. 

Because Guam is an unincorporated United States territory, Katrina is not entitled to receive SSI benefits. Does this law—in place since 1972—run afoul of the equal protection clause of the Fifth and Fourteenth Amendments of the U.S. Constitution?

Thanks to some extraordinary lawyering by a pro bono team from Kirkland & Ellis led by Mike Williams and Susan Davies, a federal judge in Guam said yes. Siding with Katrina, Chief U.S. District Judge Frances Tydingco-Gatewood ruled on summary judgment that "the equal protection guarantees of the Fifth Amendment forbid the arbitrary denial of SSI benefits to residents of Guam."

If the ruling stands, it will mean about 24,000 people in Guam will be able to receive potentially life-changing disability benefits.

It's an outcome few might have predicted six years ago when Rodney Jacob, a leading lawyer in Guam and name partner at Calvo, Fisher & Jacob, first approached Williams about the case. The biggest obstacle? Two unfavorable U.S. Supreme Court decisions that seemed directly on point.

In Califano v. Gautier Torres, a man who received SSI benefits while living in Connecticut had them discontinued when he moved to Puerto Rico. He sued in U.S. District Court in Puerto Rico, claiming that the exclusion of Puerto Rico from the SSI program was unconstitutional.

In its 1978 decision, the Supreme Court ruled against him, noting that "Congress has the power to treat Puerto Rico differently, and that every federal program does not have to be extended to it."

So that wasn't helpful.

Two years later, the high court rebuffed a similar suit—a class action involving payment amounts under the Aid to Families with Dependent Children program. 

The Supreme Court pointed out that "Puerto Rican residents do not contribute to the federal treasury; the cost of treating Puerto Rico as a state under the statute would be high; and greater benefits could disrupt the Puerto Rican economy."

All of which could also be said about Guam, the Social Security Administration argued in Katrina Schaller's case.

"Binding Supreme Court precedent makes clear that Congress can pass economic and social welfare legislation affecting U.S. Territories so long as it has a rational basis for its actions," wrote Daniel Riess, a trial lawyer in the Justice Department's Civil Division. 

"Here," he continued, "the legislation at issue clearly satisfies rational-basis review, as residents of Guam do not pay federal income tax, which funds the SSI program; because the increased cost to the federal treasury of extending SSI benefits to residents of Guam would be very substantial, especially in light of the fact that Guam residents are exempted from paying federal income tax; and because Congress could have reasonably concluded that extending SSI benefits eligibility to Guam residents could result in appreciable inflationary pressure."

Lesser lawyers might have said Katrina's case was hopeless. But not team Kirkland, which in addition to Williams and Davies also included partner Beth Dalmut and associates Julia Choi, Katherine Epstein, Emily Merki Long, Luke McGuire, Paul Quincy and Paul Suitter.

Williams, a well-known trial lawyer and appellate advocate who also serves on Kirkland's firmwide pro bono management committee, was quick in an interview to credit Davies, whom he called "a certifiable legal genius," for figuring out a winning approach.

The key? The Northern Mariana Islands—a chain of 14 islands with a population of 53,883. A mere 60 miles north of Guam, the islands have been a U.S. commonwealth since 1975.

By contrast, Guam has been a U.S. territory since 1898 and its residents and have enjoyed U.S. citizenship since 1950. 

There's no question Guam has a deeper relationship with the United States, Williams said. And yet, residents of the Northern Mariana Islands are entitled to receive SSI benefits while those in Guam are not. 

How is that fair?

"Our argument wasn't just Guam versus the United States, it was Guam versus the Northern Mariana Islands," Williams told me. 

By re-focusing the case on the disparate treatment of Guam and the islands, the Kirkland team was able to argue that reliance on the unfavorable Supreme Court decisions was misplaced.

The judge bought it. 

"While Guam's tax status might explain why it is treated differently from the fifty States and the District of Columbia, it does not justify the distinction in treatment between Guam and the [Northern Mariana Islands] with regard to SSI benefits," Tydingco-Gatewood wrote. 

Nor would extending SSI benefits to Guam break the U.S. Treasury, she found. Costs estimates range from $17 million to $175 million—or 0.03% to 0.3%. of the SSI program's $54 billion expenditures in 2017.

"As plaintiff notes, such a minimal increase in cost does not qualify as 'extremely great' so as to justify the unequal treatment of eligible citizens residing in Guam," she wrote.

As for the economic disruption argument, well, that's just stupid.

Guam residents in need can already get SNAP and Medicare benefits, "and there is no evidence to suggest that the influx of these federal funds have negatively impacted Guam's economy. To the contrary, these public assistance dollars from the federal government have benefitted Guam's economy," the judge wrote. "[I]t is irrational to conclude that Guam's economy would be disrupted if it were included in the SSI program."