Congress enacted a wide range of reforms that promised to restrain presidential power. The War Powers Act of 1973 created mechanisms to ensure that Congress authorized the deployment of American troops abroad. The Budget Reform of 1974 centralized the process used by the House and the Senate to make decisions about spending money so as to make the legislative branch more of an equal of the executive. The Campaign Finance Reform Act of 1974 established a system of public finance for presidential elections along with spending and contribution limits.

The National Emergencies Act of 1976 authorized the president to initiate emergency powers of government, but with the condition that he or she specify the particular provisions in detail. (This is the authority President Trump has reportedly explored as a way of funding his wall.) Intelligence reforms imposed limits on the C.I.A. and F.B.I., whose surveillance and national security operations had greatly enhanced the president’s power. Last but not least, the Ethics in Government Act of 1978 sought to watch against a replay of Watergate by establishing the Office of the Independent Counsel to ensure that there would be independent investigations into executive branch corruption.

Despite these reforms, four decades later, the “imperial presidency” still seems to be alive and well. What went wrong?

The most familiar challenge stems from the fact that in the midst of national security crises, much of the nation remains willing to allow presidents to respond to its perceived enemies. Despite the War Powers Act and the larger lessons of Vietnam, Congress has continued to allow presidents to send troops into combat without a formal declaration of war. In response to the terrorist attacks of Sept. 11, Congress passed legislation authorizing a vast expansion of the national security system that gave President George W. Bush and his successors access to new organizations, programs and institutions through which to pursue national security goals without congressional support.

Since the 1970s, Democrats and Republicans have sorted themselves by party, with less room for internal dissent and less of a will to criticize or challenge a president from one’s own party. Both parties have been willing to grant the president more authority when it served their purpose. The main dynamic for Democrats has centered around party leaders supporting presidents who use executive action, through regulatory orders and rule making, to deal with urgent policy problems that congressional Republicans oppose. Presidents Jimmy Carter and Bill Clinton turned to executive power to deal with energy and climate change when Congress refused to do so. President Barack Obama did the same on immigration after congressional obstruction prevented compromise legislation from passing.