On perhaps the defining issue of the 2016 Republican primary, Senator Ted Cruz falls well to the right of Ronald Reagan, who supported granting legal status to millions of undocumented immigrants.

He opposes abortion even in cases of rape and incest, and has called for a federal amendment that would allow states to avoid performing or recognizing same-sex marriages.

He wants to return to the gold standard, abolish the Internal Revenue Service and create a tax structure simple enough for Americans to file on postcards.

He has criticized Donald J. Trump on deportation policy. From the right.

Throughout his Senate career, Republican opponents have cast Mr. Cruz as a master of the ill-considered — a “wacko bird,” as Senator John McCain of Arizona once called him — whose seemingly reckless pursuits were thought to place him well outside the mainstream.

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.”

There have at times been perceptible shifts from Mr. Cruz during the campaign, in both tone and substance, coaxed by the resonance of Mr. Trump’s populist anger and hard-line positions on trade and immigration.

But at its core, Mr. Cruz’s brand of conservatism is the product of decades of careful study and manifest intellectual firepower, fusing a host of historical strands into what he has called “opportunity conservatism.”

As a teenager, growing up in Houston, he earned money delivering speeches on Friedrich A. Hayek and Ludwig von Mises, expounding on free-market principles at Rotary and Kiwanis clubs.

The epigraph for his senior thesis at Princeton, which focused on states’ rights and the Ninth and 10th Amendments, quoted James Madison: “You must first enable the government to control the governed; and in the next place oblige it to control itself.”

When he captured the Republican nomination in his 2012 Senate race, Mr. Cruz said he was “walking in Uncle Milton’s footsteps,” to honor the 100th birthday of the economist Milton Friedman.

He is fond of invoking Mr. Reagan’s Cold War dictum (“We win, they lose”), Margaret Thatcher’s dismissal of socialism (“The problem with socialism is, eventually you run out of other people’s money”), and even, at times, President John F. Kennedy.

“I intend to have in the office of president what J.F.K. used to refer to as ‘vigaaaahhh’ in defending the Constitution,” Mr. Cruz, now 45, told voters in Iowa in January.

Some citations are more familiar to conservative audiences than others.

On economic policy, he has at moments turned to Ayn Rand, the libertarian heroine lionized by the right, and John Rawls, the liberal political philosopher who argued for a compact protecting the “least advantaged.”

References to Mr. Rawls have dwindled since Mr. Cruz began his presidential candidacy last year. “I don’t think Rawls focus-groups in Iowa,” Mr. Ponnuru joked.
It is clear that Mr. Cruz, whose campaign did not make him available for an interview, is most animated by constitutional fights over what he views as overreach by the federal government, particularly on matters of religion. He speaks often of his triumphs as solicitor general of Texas, which included the successful defense of the state’s right to display a Ten Commandments monument at the Capitol.

He is a creature of the Supreme Court, counting Chief Justice William H. Rehnquist as a former boss and Justice Antonin Scalia as a friend whose strict constructionist views helped shape his own.

And arguing last year against a federal ban on marijuana — despite his personal opposition to marijuana legalization at the state level — Mr. Cruz recited Justice Louis D. Brandeis’s belief in the states’ role as “laboratories of democracy.”

Indeed, conservative thinkers have sensed in Mr. Cruz an array of less likely forebears: the faith-focused morality of Jimmy Carter or George W. Bush; President Obama’s disdain for Beltway think-tank consensus; the fictional exploits of Frank Capra and Jimmy Stewart.

“There’s a little bit of ‘Mr. Smith Goes to Washington,’” said Peter D. Feaver, a national security strategist under Mr. Bush and a political-science professor at Duke University. “The, ‘I’m going to pursue this even if everyone else is mad at me because this is right.’”

Mr. Cruz seems to take uncommon pride in communicating uncomfortable policy positions to potentially hostile audiences. Mr. Ponnuru likened Mr. Cruz’s opposition to ethanol subsidies in Iowa — once considered heretical in a presidential primary — to Mr. Goldwater’s unpopular suggestion in 1964, while campaigning in Tennessee, that the Tennessee Valley Authority should be sold.

On matters of foreign policy, Mr. Cruz is viewed much more warily by mainstream Republicans. His pre-Senate career dealt little with international affairs, many say, and his first term has contained some notable shifts.

Mr. Cruz entered the Senate in 2013 as part of the Tea Party wave, brandishing a libertarian streak that became more pronounced after revelations of government surveillance tactics, courtesy of Edward J. Snowden.

But in the years since, as the national dialogue has grown more consumed by security threats like the Islamic State, Mr. Cruz has recalibrated considerably, leaving an impression among some conservative thinkers that he is merely groping for the median position of the base.

Most notable during the campaign has been his pledge to “carpet-bomb ISIS into oblivion,” which has earned a rebuke from military leaders who define the term specifically as the blanket bombing of even civilian areas. Mr. Cruz has argued his iteration can be more targeted.

“He means an overwhelming air campaign,” his longtime national security adviser, Victoria Coates, clarified in an interview.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center who served under Mr. Reagan and Presidents George Bush and George W. Bush, said Mr. Cruz had been “a little hard to find on the spectrum” of conservative foreign policy.

“The latest incarnation for him is a sort of realist school of overwhelming military force, but he’s constantly criticizing nation-building,” Mr. Wehner said. “I have a feeling he’s more of an amateur in that area.”

The introduction of a national security team last month failed to inspire universal confidence: While the list included some well-respected members of the Reagan and Bush administrations, it also had Frank Gaffney Jr., viewed by many as a fringe conspiracy theorist who has suggested that President Obama is Muslim.

“Being critical of Frank is a cottage industry,” said Ms. Coates, whose own résumé — she is better known as an art historian — has been questioned. “The fact of the matter is, he has been one of the few fearless voices speaking out against the problems of radical Islam.”

Other concerns are more semantic. Mr. Cruz has been criticized for appearing to use “neo-con” as a pejorative, and for characterizing his foreign policy views as falling “somewhere in between” two polar extremes: the libertarianism of Senator Rand Paul of Kentucky and the hawkishness of Mr. McCain or Senator Lindsey Graham of South Carolina.

He speaks often of Reagan-style “peace through strength,” reminding crowds that the largest country Mr. Reagan invaded was Grenada.

But some conservative foreign policy experts see a crucial difference, implicit in a February speech outlining his military plans in South Carolina.

“Cruz’s defense speech was couched as being Reaganite, with plans to increase military spending substantially,” said Gary J. Schmitt, a resident scholar at the American Enterprise Institute. “But the caveat was getting the economy fixed first. Reagan increased defense spending even while deficits soared.”

Mr. Wehner worried generally that Mr. Cruz had displayed an “intellectual rigidity” that afforded him little latitude to adapt.

Mr. Cruz has long trumpeted his “consistent conservative” credentials as a strength, proudly recalling his teenage days as part of a team of students who toured event spaces in Texas and wrote out the Constitution on easels.

Robert P. George, his mentor and thesis adviser at Princeton, said that he was most struck by “the consistency from the time when he was a student to now.” Many conservatives cite Madison and Tocqueville, he said, but “Ted has actually read them.”

Mr. George’s most memorable lesson was in humility. In his book, Mr. Cruz writes of receiving a graded paper from Mr. George, seeing a “C+” on a folded corner of the first page and panicking. “With white knuckles, I folded the corner over,” he wrote, “and on the front was written, ‘Just kidding! A.’”

Mr. George’s goal was simple. “I thought he should at least have a few moments’ experience,” he said, “of not being the smartest guy in the class.”

Ted Cruz’s Conservatism: The Pendulum Swings Consistently Right