Is the Separation of Powers the ‘republican remedy for the diseases most incident to republican government’ that Madison supposed?

When the political leaders and statesmen collectively known as the Founding Fathers of the United States of America met in Philadelphia in 1787 to address the problems in governing the country, they sought to establish a limited but effective federal republic within a constitutional framework that would stand the test of time. In defence of the principles established in the Constitution, James Madison composed Federalist no.10 - “one of the republic’s holy texts” (Runciman: 2006) - in which Madison examines how the vital constitutional principle of Separation would provide a bulwark against tyranny both of the majority and of one over-powerful branch of government, controlling the violence and damage caused by factions. Stemming from Montesquieu’s theory that “when the legislative and executive powers are united in the same person or body… there can be no liberty” as well as Madison’s view that “the accumulation of all powers legislative, executive and judiciary in the same hands… may justly be pronounced the very definition of tyranny”, the US Constitution upholds the Separation of Powers, according to which the three branches of the federal government ought to remain separate and distinct. Built into this framework was a degree of conflict between the branches, a system of Checks and Balances through which Madison believed “Ambition [would] counteract ambition” (1787). This stands in contrast to the UK’s system of Fusion under which members of the executive are all and must be members of the legislature, occasionally leading to that which Lord Hailsham coined “elective dictatorship” (1976) when the British executive dominates parliamentary proceedings with a workable majority. Under Separation, the three branches of American government are constrained by their enumerated powers, and power is reserved to the states in Amendment X.
To the extent that Separation avoids the 'elective dictatorship' that Hailsham astutely observed in British government, it is indeed a successful remedy to this day. For example, while Donald Trump’s populist, zero-sum ‘ethnic nationalism’ has proceeded to breach the US’s obligations under international law not to discriminate against refugees on grounds of religion, under the very principles that Madison established, the court refused to reinstate his executive order banning refugees and restricting travel to the United States from seven Muslim-majority countries. Nonetheless, the historical principle has perhaps led to new 'diseases', two of which are vital to consider. First, it has not only failed to prevent, but perhaps fostered the rise of the 'imperial presidency' since Franklin D. Roosevelt came to office, particularly during periods of united government, when two or more branches are of the same party majority. This was seen under the Bush administration 2002-2006 when Washington became a ‘one party town’ and civil rights were curtailed under the “dispensation of a new world order” (Runciman: 2006). Equally, the presence of parties can also result in divided government leading to gridlock, during which a breakdown in the working arrangements between the branches of government occurs, resulting in real difficulties in passing legislation and implementing policies. The second vital 'disease' to consider is the rise of the imperial judiciary. Since 2005, Chief Justice John Roberts’ race-based agenda has seemingly led the court’s conservative majority against measures which protect the voting rights of African-Americans. In Shelby County vs Holder (2013) the Supreme Court struck down Section 4(b) of the 1965 Voting Rights Act as unconstitutional, on the grounds that it imposed burdens that were no longer responsive to modern conditions in the voting districts in question. This means that until Congress passes legislation with a new formula for preclearance under Section 4, jurisdictions that were covered by the previous formula are free to make election changes without any need for approval from the federal government. In the highly controversial ruling of Citizens United vs FEC (2010), First Amendment Rights were extended to corporations. The constitution, in instances such as these, becomes “a mere thing of wax in the hands of the judiciary” (Jefferson: 1803) as do the rights of minorities. Here, the prevailing judicial philosophy of the court assumes profound political significance, therefore even if the principle of Separation is informed by a counter-majoritarian spirit, John Roberts continues to lead the court’s conservative majority in what appears to be “a counterrevolution against the fruits of the civil rights movement” (Patterson: 2016).