To Mr. 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, Mr. 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. (Mr. 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.