Presidential candidate Sen. Rand Paul (R-Kentucky) at the First in the Nation Republican Leadership Conference in Nashua, New Hampshire, on April 18. Reuters It turns out that Republicans in Washington are united on one issue: their hatred of Rand Paul. John McCain says Paul is "the worst possible candidate . . . on the most important issue."

Marco Rubio opines that Paul "has no idea what he's talking about."

Lindsey Graham concludes that it would be "devastating" for the party to nominate him.

Conservative commentators are even more vicious and ad hominem.

The obsession with Paul is striking. In a Post op-ed article last summer, then-Texas Gov. Rick Perry mentioned Paul 10 times. I cannot recall an instance in recent decades in which so much vitriol has been directed against a leading political figure by his own party.

The attacks are almost entirely focused on Paul's foreign policy, which is routinely characterized as dangerous and isolationist. In fact, the real problem appears to be that Paul is trying to force Republicans and many Democrats to defend what has become a lazy, smug consensus in favor of an ever-expanding national security state.

I have read Paul's proposals and speeches on foreign policy. There are some bloopers, odd comments, and rhetorical broadsides, but for the most part his views are intellectually serious and well within a tradition of what he (correctly) calls conservative realism.

They are also politically courageous. Paul has taken positions and cited authorities that are deeply unpopular with his own party.

Yes, of course, he craves publicity and engages in stunts. What politician doesn't? But what makes his opponents most uncomfortable is the substance, not the style.

Sen. Rand Paul (R-Kentucky). AP/Cliff Owen Take the most recent example: his opposition to the blanket extension of the Patriot Act, which has resulted in some modest restraint on the vast expansion of government powers since 9/11. (The new checks and balances are close to ones recommended by a panel put together by the Obama administration.)

In defending his position, Paul notes — correctly — that we would not even know of the existence of this system of metadata collection if not for Edward Snowden's revelations, that the FBI has been unable to cite a single terrorist plot disrupted by it, and that the special courts in place have few checks and little transparency.

He cites, glowingly, the 1979 dissenting opinion regarding the dangers of government collection of phone records by Thurgood Marshall and William Brennan, the Supreme Court's two most prominent liberals of the past half-century.

Or consider Paul's views on lifting the embargo on Cuba, on which he writes: "The supporters of the embargo . . . fall strangely silent when asked how trade with Cuba is so different than trade with Russia or China or Vietnam." This is not a path to primary voters' hearts in Florida.

He has raised uncomfortable questions that no other politician dares raise about Anwar al-Awlaki, an Al Qaeda leader who was killed in a car on a road in Yemen by a US drone strike. Paul has pointed out that since Awlaki was a US citizen, this action creates an extraordinary legal precedent — that the president of the United States can execute a US citizen without trial. He cites approvingly the American Civil Liberties Union, which, he writes, has pointed out that "in modern history, a presidential order to kill an American citizen away from a battlefield is unprecedented."

Sen. Rand Paul (R-Kentucky). AP/Jim Cole In the Middle East, Paul has called for caution before the next military intervention, suggesting that it is worth learning some lessons from the past decade. US military interventions, he has argued, have destabilized countries and led to perverse consequences. "As secular dictators fell in Libya, Egypt, Iraq, and now Syria, radical jihadists exploited the vacuum," he has noted.

In Afghanistan, he said, President Obama added 50,000 troops to the US force and spent an additional $120 billion on the effort with little effect. Afghanistan today is by some measures as dangerous as ever — after 14 years of continuous US military intervention and $1 trillion spent, by one estimate. Surely this is worth pondering?

I don't agree with Rand Paul on many things, including foreign policy. I think some of his positions on civil rights are historically blind, cruel, and dangerous. But in the arena of national security, he has time and again raised important, inconvenient questions, only to have them ruled out of order and to be told that he is a crank, far outside the mainstream. In fact, it would be useful and important for Republicans — and Democrats — to stop the name-calling and actually discuss and debate his ideas.