With a blitz of moves in his 100 days in office, President Trump has sought to greatly enlarge executive power. The typical explanation is that he’s following and expanding a legal idea devised by conservatives during the Reagan administration, the unitary executive theory.

It’s not even close. Mr. Trump has gone beyond that or any other mainstream notion. Instead, members of his administration justify Mr. Trump’s instinctual attraction to power by reaching for a longer tradition of right-wing thought that favors explicitly monarchical and even dictatorial rule.

Those arguments — imported from Europe and translated to the American context — have risen to greater prominence now than at any time since the 1930s.

Mr. Trump’s first months back in office have provided a sort of experiment in applying these radical ideas. The alarming results show why no one in American history, up until now, has attempted to put them into practice — and why they present an urgent threat to the nation.

The tradition begins with legal theorist Carl Schmitt and can be followed in the work of the political philosopher Leo Strauss, thinkers affiliated with the Claremont Institute, a California-based think tank with close ties to the Trump movement, and the contemporary writings of the legal scholar Adrian Vermeule. Many on the right have bristled at presidential power being constrained over the past century by two waves of administrative reform. The first dates back to the early 20th century and the rise of the bureaucratic-regulatory state during the Progressive and New Deal eras. The second wave emerged in the 1970s, as Congress responded to the abuses of power by Richard Nixon.

The presidency has evolved to become an office exercising general (and often passive) oversight of vast departments and agencies, which are staffed by career civil servants who stay on across administrations.