Republicans Chris Christie and Rand Paul are slinging mud far ahead of the 2016 presidential election. We take a look into one of Christie's claims. (The Washington Post)

Way back in 1975, a Republican agitator named Ronald Reagan had this to say about an esoteric young movement that was roiling politics: “If you analyze it, I believe the very heart and soul of conservatism is libertarianism.”

Neither the GOP old guard nor the rowdy libertarians ever quite bought that argument.

They both lay claim to the same conservative economic philosophy. But libertarians are more isolationist and antiwar than Republican orthodoxy allows on foreign policy and more permissive on social issues.

Still, in the nearly four decades since Reagan made those comments, the two have managed — at least most of the time — to maintain an uneasy marriage of expedience.

Libertarianism once again appears to be on the rise, particularly among the young. But its alliance with the Republican establishment is fraying, as demonstrated by the increasingly personal war of words between two leading potential 2016 presidential contenders.

The sparring began last week, when New Jersey Gov. Chris Christie (R) posited: “As a former prosecutor who was appointed by President George W. Bush on Sept. 10, 2001, I just want us to be really cautious, because this strain of libertarianism that’s going through both parties right now and making big headlines, I think, is a very dangerous thought.”

After Christie made it clear that he was referring to Rand Paul, the Senate’s leading critic of the National Security Agency and its surveillance programs, the Kentucky Republican fired back on his Twitter account: “Christie worries about the dangers of freedom. I worry about the danger of losing that freedom. Spying without warrants is unconstitutional.”

Their feud — which is being watched closely as a possible warmup round for 2016 — has continued, expanded and spilled over into other issues.

On Tuesday, Christie chided: “If Senator Paul wants to start looking at where he’s going to cut spending to afford defense, maybe he should start cutting the pork-barrel spending that he brings home to Kentucky.” After which Paul told CNN that the plus-size governor was “the king of bacon talking about bacon.”

This kind of rancor is pretty much the last thing the Republican Party needs right now as it struggles to broaden its appeal and find its footing in the wake of two successive presidential defeats.

For their part, libertarians are thrilled. They say it is a sign they truly have arrived as a force to be dealt with, rather than dismissed as a fringe element.

“There are a lot of people within establishment Republican Party politics who have controlled the process for the last 10 or 20 years who fear that their grip on the party is slipping away,” said Rep. Justin Amash (R-Mich.), whose amendment to restrict the NSA’s ability to collect telephone records came surprisingly close to passing in the House last week.

In an interview, Amash argued that despite opposition from House GOP leaders, his point of view represents an advancing wave among House Republicans. He cited an analysis by Bloomberg News showing that while House Republicans who have served more than five years opposed his amendment by more than 2 to 1, it won a slim majority among those who have arrived there more recently.

The amendment, which was co-authored by Rep. John Conyers Jr. (D-Mich.), a liberal stalwart who is a half-century older than Amash, also won more than 100 Democratic votes.

Libertarianism tends to rise as a backlash to government expansiveness and incompetence, said David Boaz, executive vice president of the libertarian Cato Institute, who has written extensively about the movement’s history.

He said the modern movement began to flower in the late 1960s and early 1970s in reaction to the Vietnam War, disenchantment with the growth of social programs during the Great Society era, stagflation and the Watergate scandal.

Libertarianism also took on an intellectual sheen after proponents Friedrich August von Hayek and Milton Friedman won Nobel Memorial Prizes in economics in 1974 and 1976, respectively, and Harvard University professor Robert Nozick’s defense of it, titled “Anarchy, State, and Utopia,” won the National Book Award in 1975.

Politically, “libertarianism with a small L was very skeptical of Republicanism with a capital R,” but they were bound by their mutual abhorrence of communism and the welfare state, Boaz said.

After the Soviet Union fell apart, their relationship became more tenuous.

In the 1992 election, for example, a Cato Institute analysis found that the 13 percent or so of voters who were libertarian-minded — those who told pollsters they wanted smaller government but tolerant social policies — split almost evenly among Republican incumbent George H.W. Bush, Democrat Bill Clinton and third-party candidate Ross Perot.

The movement these days has been galvanized in part by the 2008 and 2012 presidential campaigns of former representative Ron Paul (R-Tex.), father of the Kentucky senator, which drew a vocal following among young people.

When libertarian Clark Ruper was a University of Michigan student from 2004 to 2007, he recalled, “there were, like, five of us on campus, and we all knew each other.”

Now vice president of a rapidly growing organization called Students for Liberty, Ruper says of the dust-up between Christie and Paul: “I think it’s fantastic. When guys like Chris Christie are attacking us, we must be doing something right.”

Ruper, for one, rejects Reagan’s depiction of conservatism and libertarianism as being one and the same.

“We are not a branch of conservatism,” largely because of social issues like same-sex marriage and drug legalization, Rupar said. “Those are real deal-breakers where we can’t get along with conservatives. We find our allies there on the left.”

Nor was Reagan himself comfortable with all the tenets of libertarianism.

“I think that, like in any political movement, there are shades, and there are libertarians who are almost over at the point of wanting no government at all or anarchy,” Reagan said in that same 1975 interview with the libertarian magazine Reason.

Libertarians also have pronounced differences with many on the right when it comes to foreign policy.

Jeff Frazee, executive director of a more politically oriented libertarian organization called Young Americans for Liberty, said that young people today are skeptical of intrusive government at home and want to see an end to mililtary intervention overseas.

“Many of the members in our group were not even 10 years old when 9/11 happened,” Frazee said. “They’ve grown up with war, and they are war-weary.”

Libertarians still count relatively few elected officials as their own.

Rand Paul comes the closest. Libertarians have cheered his stance on surveillance and his 13-hour filibuster in March to protest the Obama administration’s use of unmanned drones.

That filibuster brought withering commentary from the conservative establishment.

“If Mr. Paul wants to be taken seriously he needs to do more than pull political stunts that fire up impressionable libertarian kids in their college dorms,” the Wall Street Journal wrote. Sen. John McCain (R-Ariz.) called Paul and his Senate allies “wacko birds.”

Yet even Paul draws some skepticism from libertarian purists. They are leery, for instance, of his recent overtures to the Christian right, a constituency he cannot afford to alienate if he hopes to win his party’s presidential nomination.

For now, however, libertarian activists seem to believe that their best shot at advancing their ideas is in changing the Republican Party from the inside.

“Where else are you going to go?” asks Brian Doherty, senior editor at Reason. “Given the shape of the culture and the ideology of American politics, the Republican Party is at least willing to pay lip service to libertarian values. It is the existing space that a libertarian can move into and sort of fit in.”