Libertarians, to the extent we can ever hope for real political success, should be excited about Rand Paul. But Paul, the Kentucky senator who is running for president, has treated his libertarian inheritance as more burden than boon. He's even called libertarianism an "albatross," though a growing, and young, share of the Republican Party is rallying around libertarian ideals.

Paul inherited the problem of libertarian purity from his father, Ron Paul, who served three stints as a congressman from Texas and sought the Republican nomination for president in 2008 and 2012.

The younger Paul campaigned for his father, but is now edging away warily. Invoking Barry M. Goldwater is hazardous for a Republican these days. Goldwater's insistence on a limited role for government seemed radical even in 1964, and yet, with enthusiastic youth activist support, the Republican Party gave him its nomination that year. (He then lost to Lyndon B. Johnson, in a landslide.)

Goldwater famously said, "Extremism in the defense of liberty is no vice." Rand Paul's defense of liberty has been lackluster, and lacking much virtue. He does go further than other Republicans when he calls for tamping down the drug war, reforming criminal sentencing and respecting states that have legalized marijuana. But he hasn't sufficiently gone after excessive restrictions on what food we can buy and what drugs we can use.

In foreign policy, Rand Paul shares with his father — who warned that American power abroad was often not a force for good — an opposition to "nation building." But rather than rethinking our entire foreign policy, Paul has proposed a two-year, $190 billion budget increase in military spending. (He has urged corresponding cuts in domestic programs and foreign aid.) He repeats the mindless mantra that "radical Islam" must be fought — but he does not adequately say how, where or at what cost.

To Paul's credit, he's suggested slashing 20 percent from the Centers for Disease Control and Prevention, shrinking the National Science Foundation by 62 percent, and taking a 25 percent chunk out of NASA, among other cuts.

But such cuts are vulnerable to endless, piecemeal tinkering that will lead only to more out-of-control spending. Instead, Paul should consistently advance the core libertarian notion that some things just aren't an appropriate function of the government at all. For a true libertarian, the government's power to tax should be used only to protect our natural rights to our lives and property.

Libertarianism's relevance to the problems that bedevil the Republican Party, and America, goes beyond spending. You can't solve our foreign policy problems until you understand that the military's purpose is to defend lives and property on the homeland — not fight international villainy. You can't solve the immigration problem unless you understand that people, like goods and services, should be allowed maximum freedom of movement. (Paul, like other Republicans, instead wants to strengthen the borders.) You can't make criminal justice truly just until you limit the reasons government fines and imprisons us to true crimes against persons or property, not minor "quality of life" infractions or life choices (like drug use) that the government simply disapproves of.

It seems Paul prefers the term "constitutional conservative" to "libertarian." But it's a fairly empty distinction. America's government was conceived as a small island of limited powers in a sea of unenumerated rights. Paul has vowed to end the National Security Agency's dragnet of phone data collection. But without arguing rigorously from a core libertarian philosophy, he will find it hard to answer the question: Why not give up some privacy in the name of "national security"? Paul needs to rigorously defend the principle that certain rights trump all other considerations — even public safety.

Do libertarian ideals sound heady, even ridiculous, to many Americans? Sure. That's precisely why we need a candidate who will articulate them. Who else will challenge both parties' complacent assumptions — and their shared devotion to ever-increasing spending on problems that exceed the proper scope of government? Since Ronald Reagan, Republicans have been as bad as Democrats in defining what government is for — what it can and can't rightly do.

Calling for some limits on government intrusion here, and some spending cuts there, isn't enough. America needs a political champion willing to stretch beyond the merely practical. It's a tricky position for anyone running for president to try to move the majority in your direction. But if liberty is your highest value, it's essential.

Brian Doherty, a senior editor at Reason magazine, is the author of "Ron Paul's Revolution: The Man and the Movement He Inspired." He wrote this article for the New York Times.