The elevator speech version is that in the postwar period we built a powerful presidency, a president-centered national government. On the institutional side, we did this because of new global commitments and conflicts, the need for management of the expanded administrative state, Congress's inability to lead in management of the national economy, and as a response to the rise of radio and television. Presidents gained the capacity for unmediated communication to the public; in mediated communication the president became the focal point for the dramas that played out in televised news.
On the political side, this all played out in a period of broad "consensus politics." Until the mid-1960s the Cold War consensus on foreign policy mostly held. Until 1980, roughly, the Keynesian consensus on economic policy held. This was one of the fundamental political conditions for the rise of the modern presidency--regardless of party presidents could be counted on to operate somewhere near the mid-point of the consensus on foreign and most domestic policies. In effect, this was a "check" on presidential power, even as that power expanded. There were other institutional and political checks. Parties still controlled nominations; the Congress still had formidable, autonomous centers of power in the committee chairs; voters didn't perceive huge differences between the parties, and many did swing and split tickets through this period.
In this era scholars of American government championed presidential power as a key to breaking gridlock on pressing issues, and it was even sort of sensible for Richard Neustadt to write, as he did in his classic Presidential Power (1960), that "What's good for America is good for the president, and vice versa."
Where are we now? The institutional conditions that fed, even required, the growth of presidential power remain, and the substantial resources controlled by the executive branch are still in place. But the political conditions that made the growth of presidential power broadly tolerable and politically manageable have collapsed. From Reagan forward, with GHW Bush as a possible exception, every president in this deeply polarized polity is perceived as a factional leader. They emerge from a fractured polity, and the exercise of their substantial powers in service of what are perceived as factional agendas drives further divisions. Thus we see sharp and growing differences in presidential approval by partisanship (see the figure below), and we see the broader manifestations in Clinton Derangement Syndrome, Bush Derangement Syndrome, Obama Derangement Syndrome, and Trump Derangement Syndrome, with all of these presidents' opponents bitterly opposed to their leadership across the board. What this "factional presidency" does to American politics is qualitatively different than the politics presidents of that earlier era made. The aggressive exercise of expansive presidential powers, long championed by liberals and conservatives alike as a necessity given the demands of foreign policy and the rise of the administrative state, now seriously threatens political stability and order. The factional presidency is a result of the many forces that have driven partisan polarization; it is also now a primary driver.
Or something like that. Here's something from Gallup on presidential approval by party, from Eisenhower's first year through Trump's first year.
