2006] SEPARATION OF PARTIES,NOT POWERS 23I5 own and he often may win,as a political leader,what he cannot command under the Constitution.8 Justice Jackson astutely recognized that the separation of powers no longer works as originally envisioned because interbranch dynamics have changed with the rise of political parties,which in Youngstown had diminished the incentives of Congress to monitor and check the President.Yet this part of Justice Jackson's opinion has been ignored entirely.Even after decades of dissecting Justice Jackson's Youngs- town opinion,neither the Supreme Court nor any other federal court has ever quoted this critical insight,nor has it received much notice by legal scholars.Justice Jackson's sophisticated realism about the workings of government is widely admired by constitutional lawyers, but his most penetrating realist insight-recognizing party competi- tion as a central mechanism driving the institutional behavior that separation-of-powers law aims to regulate-has been missed. This Article seeks to recover and build upon Justice Jackson's in- sight,reenvisioning the law and theory of separation of powers by viewing it through the lens of party competition.We emphasize that the degree and kind of competition between the legislative and execu- tive branches vary significantly,and may all but disappear,depending on whether the House,Senate,and presidency are divided or unified by political party.The practical distinction between party-divided and party-unified government rivals in significance,and often dominates, the constitutional distinction between the branches in predicting and explaining interbranch political dynamics.Recognizing that these dy- namics shift from competitive when government is divided to coopera- tive when it is unified calls into question many of the foundational as- sumptions of separation-of-powers law and theory.It also allows us to see numerous aspects of legal doctrine,constitutional structure,com- parative constitutionalism,and institutional design in a new and more realistic light. The Article proceeds as follows.Part I lays out the conceptual case for switching the focus of separation of powers from branches to par- ties,arguing that political competition in government often tracks party lines more than branch ones.Recognizing that party competi- tion can either create or dissolve interbranch competition,depending on whether government is unified or divided by party,suggests that the United States has not one system of separation of powers but (at least)two.Part II looks more closely at these dual-party-divided 8 Id.at 654 (Jackson,J.,concurring). 9 It is cited,in passing,in two law review articles.See Norman C.Bay,Executive Power and the War on Terror,83 DENV.U.L.REV.335,345 (2005);Linda S.Greene,After the Imperial Presidency,47 MD.L.REV.99,106 (1987).2006] SEPARATION OF PARTIES, NOT POWERS 2315 own and he often may win, as a political leader, what he cannot command under the Constitution.8 Justice Jackson astutely recognized that the separation of powers no longer works as originally envisioned because interbranch dynamics have changed with the rise of political parties, which in Youngstown had diminished the incentives of Congress to monitor and check the President. Yet this part of Justice Jackson’s opinion has been ignored entirely. Even after decades of dissecting Justice Jackson’s Youngs￾town opinion, neither the Supreme Court nor any other federal court has ever quoted this critical insight, nor has it received much notice by legal scholars.9 Justice Jackson’s sophisticated realism about the workings of government is widely admired by constitutional lawyers, but his most penetrating realist insight — recognizing party competi￾tion as a central mechanism driving the institutional behavior that separation-of-powers law aims to regulate — has been missed. This Article seeks to recover and build upon Justice Jackson’s in￾sight, reenvisioning the law and theory of separation of powers by viewing it through the lens of party competition. We emphasize that the degree and kind of competition between the legislative and execu￾tive branches vary significantly, and may all but disappear, depending on whether the House, Senate, and presidency are divided or unified by political party. The practical distinction between party-divided and party-unified government rivals in significance, and often dominates, the constitutional distinction between the branches in predicting and explaining interbranch political dynamics. Recognizing that these dy￾namics shift from competitive when government is divided to coopera￾tive when it is unified calls into question many of the foundational as￾sumptions of separation-of-powers law and theory. It also allows us to see numerous aspects of legal doctrine, constitutional structure, com￾parative constitutionalism, and institutional design in a new and more realistic light. The Article proceeds as follows. Part I lays out the conceptual case for switching the focus of separation of powers from branches to par￾ties, arguing that political competition in government often tracks party lines more than branch ones. Recognizing that party competi￾tion can either create or dissolve interbranch competition, depending on whether government is unified or divided by party, suggests that the United States has not one system of separation of powers but (at least) two. Part II looks more closely at these dual — party-divided ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– 8 Id. at 654 (Jackson, J., concurring). 9 It is cited, in passing, in two law review articles. See Norman C. Bay, Executive Power and the War on Terror, 83 DENV. U. L. REV. 335, 345 (2005); Linda S. Greene, After the Imperial Presidency, 47 MD. L. REV. 99, 106 (1987)