More than any other president in recent memory, Donald Trump seems uninterested in being a president for all Americans. His hyperpartisan language (“They are sick, radical left people”) makes this clear. So does his use of the levers of power — his abuses of the pardon power, for example, and the extraordinary transformation of the Justice Department.

But it would be a mistake to see his approach as simply the result of a vindictive president who swats away norms and embraces hardball politics. Mr. Trump’s outsize persona has given cover to an extraordinary ideological radicalism in the Republican Party. The excesses of the second Trump administration would not be possible without the intellectuals who have gathered in the MAGA movement. To really understand what is happening in the United States today, we must understand the ideology and thinkers behind the MAGA New Right.

Mr. Trump’s approach to governing almost exclusively for red America dovetails with the closed philosophical mode of this group, which embraces radical anti-modernism and tends toward moral and political absolutism. In this melding, only one political party possesses truth and reality — and governs accordingly. The MAGA New Right’s contempt for liberal democracy is rooted in this alternate vision, and it rejects the various forms of pluralism and tolerance that many Americans have taken for granted.

Many supporters of the president understand themselves to be the vanguard of an ambitious project that will continue to shape G.O.P. politicians and policy long after Mr. Trump leaves office. Some names have become familiar, such as Kevin Roberts of the Heritage Foundation, Patrick Deneen at the University of Notre Dame and Yoram Hazony, an Israeli political theorist and a leading advocate for national conservatism.

But others are more obscure yet as important to understanding what’s happening in the United States. One is Richard Weaver, who wrote a book in 1948 that describes the basic contours of the New Right’s closed philosophical approach. The title of Dr. Weaver’s book, “Ideas Have Consequences,” would soon become a popular catchphrase among conservatives. It captures the spirit of the MAGA mind and the counterrevolution that we see unfolding before us.

Dr. Weaver didn’t have just any old ideas in mind: The ideas he was concerned with were distinctively modern ideas, and the consequences of these ideas were devastating. They had caused nothing less than “the dissolution of the West.”

Americans might be used to hearing conservatives blame postmodernism and critical race theory for social problems. Dr. Weaver, who died in 1963, took aim at a philosophical concept called nominalism, the rise of which he traced to early modernity. (Think of philosophers like Francis Bacon, Thomas Hobbes and John Locke.) Nominalism involves the rejection of universal concepts and absolute truths — including transcendental moral truths. Nominalists believe that truth is embedded in the particulars of the world around us. There is no universal objective moral reality as Plato and other philosophers believed and it does not exist as an expression of the divine.

Dr. Weaver insisted that nominalism was not merely wrongheaded; it was the source of all our woes. In his introduction to “Ideas Have Consequences,” he called the shift to nominalism evil and likened it to Macbeth’s seduction by “the witches on the heath.” Like Macbeth, Dr. Weaver wrote, “Western man made an evil decision, which has become the efficient and final cause of other evil decisions.” By challenging the idea of universal objective moral reality, modern man had succumbed to individualism, relativism, materialism, historicism and politics as will to power.

In my research on the MAGA New Right and in the countless hours I’ve spent in conservative academic circles, I’ve heard this Weaver-esque refrain again and again. It is hard to think of a single significant thinker of the MAGA New Right who would disagree with his assessment of the ways in which modern thought is inherently corrosive or who would dissent from his insistence that we must restore some kind of transcendental moral orthodoxy to our politics.