Rick Pildes tells you which side of the ideological divide is most responsible for its existence:
It is widely recognized that the expansion of presidential power from the start of the twentieth century onward has been among the central features of American political development. While Andrew Jackson, with his rhetorical creation of the “plebiscitary presidency,” and Abraham Lincoln, with his invocation of presidential war powers during the existential military threat of the Civil War, were among the most powerful and activist of all presidents, the nineteenth-century presidency was essentially a narrowly understood office that presided over a highly decentralized and fragmented political system. What Theodore Roosevelt later began identifying and celebrating as the “Jackson-Lincoln” school of presidential practice remained latent through most of the nineteenth century. As the timing of Roosevelt’s comments signals, it was the Progressive movement, first at the state and then at the national level, that turned to executive power as the institutional vehicle through which to bypass corruption-plagued, paralyzed legislative bodies and status quo-affirming courts, and realize the Progressives’ agenda of an activist government, responsive to average voters, that would ensure health, safety, and economic fairness in a world transformed by industrialization and concentration of economic power.
A string of Progressive Era presidents and intellectuals revived, enhanced, legitimated, and institutionalized the expansive presidency with which, with ebbs and flows, we have since lived. Woodrow Wilson, in his later years as a scholar before assuming office, urged presidents to view their office as “anything [they have] the sagacity and force to make it.” Herbert Croly, a key architect of the Progressive movement, has been characterized as seeking to realize “Jeffersonian ends through Hamiltonian means.” Indeed, this renaissance of Alexander Hamilton as the original visionary of the energetic President, capable of cutting through factional division and corruption, was characteristic and oft repeated. Calling Hamilton “the most brilliant American statesman who ever lived, possessing the loftiest and keen-est intellect of his time,” Roosevelt conjured up Hamilton’s spirit; even Roosevelt’s more conservative successor, William Howard Taft, similarly praised Hamilton as “our greatest constructive statesman.” Meanwhile, Progressives disparaged the Constitution’s system of checks and balances as a blueprint for government “divided against itself,” a government “deliberately and effectively weakened,” that could be forged into an instrument of effective power only through the dominating, energetic leadership of a commanding President.
Thus, long before the New Deal, those seeking an activist national government had envisioned a powerful presidency as the vehicle through which their aims could (and had to) be realized. In the aftermath of World War II, Congress’s power was further discredited in foreign affairs and military matters by its abject failure in the 1930s to come to terms with the threat that the rise of Nazi Germany posed – a failure that continued to limit Congress’s credibility in these areas for thirty or so years after the war. And as is well known, the ensuing rise of the Cold War, the national security state, and the constant specter of instant nuclear annihilation further enhanced the legitimacy (and reality) of ever-expanding presidential power.
Yes, conservatives eventually became fans of the imperial presidency too. But that was a reaction to the liberals. Conservatives, to paraphrase Billy Joel, didn’t start this particular fire.