But if Roosevelt was a progressive, he was not a particularly liberal one, especially by today’s standards. He had little patience for pluralism — he derided what he deemed “hyphenated Americans” — and he believed that America’s future depended on constructing a unified, common culture, a call that echoes strongly among those pushing for a new conservative nationalism today.

At the National Conservatism Conference, Rich Lowry, the editor of National Review, picked up on one of Roosevelt’s core themes in his speech, “Why America Isn’t an Idea.” Like Mr. Lowry, Roosevelt believed that America’s political culture was rooted not in universal principles about things like liberty and individual rights, but in the specifics of its history, its landscape and the experience of its people — which to Roosevelt meant predominantly Northern Europeans.

That’s one reason Roosevelt, like many of today’s conservative nationalists, endorsed restrictions on immigration. In his 1905 message to Congress, he said: “It will be a great deal better to have fewer immigrants, but all of the right kind, than a great number of immigrants, many of whom are necessarily of the wrong kind.”

Especially in his later years, Roosevelt’s nationalism, already problematic, became overtly racist. He proposed subsidies for white Americans to have more children and endorsed sterilizing the poor and mentally handicapped — a eugenic natalism that Senator Hawley writes in his book, “Theodore Roosevelt: Preacher of Righteousness,” was “not entirely dissimilar to that pursued by the German Third Reich.”

In the book, Mr. Hawley takes pains to criticize Roosevelt for his racism, which he concedes was central to Roosevelt’s vision for America, and not just an artifact of his time and place. Roosevelt’s welfare-state agenda, he writes, was not an end in itself; it was a means to facilitate the growth of a culturally homogeneous nation, dominated by the descendants of Anglo-Saxon settlers. Ms. Warren may dream of having a trustbuster as a running mate, but probably not one who refers to white people as the “forward race.”

Still, Roosevelt’s nationalism presents an even bigger problem for the right. Like Roosevelt, Mr. Hawley and others at the National Conservatism Conference endorsed a long list of ideas to facilitate a common American culture — breaking up big business, protecting domestic labor, rebuilding heartland economies.

But all of that requires a powerful, centralized federal government, rooted in an expansive executive branch — the kind that Roosevelt favored and Ms. Warren supports, but that sits at odds with the main currents of national conservatism and its belief that the federal government has been captured by “cosmopolitan” elites.