Thornburgh, a long-time supporter of self-determination, authored "Puerto Rico's Future: A Time to Decide" in 2007, in which he calls for immediate change in the island's territorial/commonwealth status. He describes it as a vestige of colonialism. He concedes, however, that change is difficult because equal segments of Puerto Ricans desire statehood or continuation of the status quo. The book is based in part on ongoing research he has done regarding Puerto Rico's vexing political status problem since he testified as attorney general on behalf of the first Bush administration on the issue before the Energy and Natural Resources Committee of the United States Senate in 1991 and for an amicus curiae brief he filed in a Puerto Rico voting rights case before the U.S. Court of Appeals for the First Circuit.