Yet a close reading of Mr. Cruz’s policy prescriptions, influences and writings over two decades, combined with interviews with conservative intellectual leaders and Cruz allies, suggest two powerful truths about the man who might yet assume the mantle of modern conservatism.

He would be the most conservative presidential nominee in at least a half-century, perhaps to the right of Barry Goldwater, testing the electoral limits of a personal ideology he has forged meticulously since adolescence.

And he has, more effectively than almost any politician of his generation, anticipated the rightward tilt of the Republican Party of today, grasping its conservatism even as colleagues dismissed him as a fringe figure.

Now, even Mr. Cruz’s staunchest Republican enemies tend to criticize him most forcefully on tactics — lamenting his leading role in the 2013 government shutdown, for instance — but not on substance, where they have generally arrived at equivalent positions.

“Nobody has been more assiduous than Cruz at staying on the same page as the conservative base of the Republican Party,” said Ramesh Ponnuru, a conservative author and senior editor of National Review, who first met Mr. Cruz when they were students at Princeton University. “That said, it was also the man meeting the moment. He was always a constitutionalist conservative, and then constitutionalism became cool among conservatives.”