Donald Trump wasn’t the only one who turned up missing in the Republican Debate in Iowa. There was also Ron Paul, the ex-congressman from Texas who’s not running this year but who, only four years ago, was polling head to head in a theoretical general election against President Obama. By far the best part of the exchange last night was when Bret Baier asked Senator Rand Paul: “So did you make a mistake by not fully — more fully — embracing your father politically at the beginning of this campaign?”

What prompted Mr. Baier’s question — one of the shrewdest of the entire race — is a campaign video of Senator Cruz saying that the Texan is, as Mr. Baier put it, “the intellectual and political heir to your father’s 2012 campaign and the liberty movement. And your father now says it’s realistic that Donald Trump will be your party’s nominee.” It happens that we’ve been waiting for months for someone to ask that question of the senator from Kentucky who spent his formative years in politics campaigning for his father.

“There’s probably no person I respect more in the country or in recent history than my father,” Rand Paul replied. “I think he was probably the most honest man in politics that we’ve ever seen in a generation. And so in no way have I ever said that I don’t embrace my father or love my father or appreciate everything that he has done for the country. I think what’s interesting about where that liberty vote goes that my father brought to the Republican Party is, I don’t think they’re necessarily going to go for Ted.”

“You know, Ted didn’t show up. We had an audit-the-Fed vote, which was the biggest thing my dad had been advocating for for 30 years. Ted didn’t have time to show up. He was the only Republican that didn’t show up for it. And so I think really that vote is going to stay in the Paul household. I think more of it is coming and it’s going to grow. The NSA is another big issue. Ted said he was for NSA reform, but then he told Marco Rubio, no, no, no, I voted for the bill because I’m for the government collecting 100% of your cell phone records.”

It happens that we spent the hour before the debate reading Ben Bernanke’s blog post warning against “Audit the Fed,” which is known as S. 2322, or the Federal Reserve Transparency Act, and is now before Senate. Mr. Bernanke makes much of the fact that what the Congress seeks is not a normal type of audit (the Fed already has Deloitte and Touche do that). Audit the Fed would set up permanent oversight by the Congress of the Fed’s formation of monetary policy, a demarche that Mr. Bernanke sees as compromising the Fed’s independence.

What a moment that could have been for Mr. Cruz, even if he couldn’t get back to vote, to give a major speech, laying this whole question out for the American people. The fact is that the congressional majority which wants this bill is one hundred percent right as rain about the constitutional question. One hundred percent of the monetary powers granted in the Constitution are granted to — and solely to — the Congress of the United States. What Ron Paul launched with Audit the Fed is a battle for a constitutional correction.

It is a battle to end the nation’s experiment in fiat money, a system of limitless dollars issued by the Federal Reserve that has created the historically high rates of un- and under-employment, of inequality unimaginable to earlier generations, and of rising personal bankruptcy rates, among other evils. It is also a battle to rein in the rapidly expanding federal government, curb the vast deficits, and start paying down the national debt. Heroic as Mr. Cruz has been in initiating the gold part of this conversation, what Ron Paul taught is the importance of sticking with it at every turn.

What a moment, too, for Senator Rubio. His contribution is, and ought to be, on the foreign policy side of this. The New York Sun agrees with Drs. Paul and Mr. Cruz right down the line on the monetary question. But we regard the debate over the Fourth Amendment as off the point; we don’t see the government’s methods of requiring metadata collection from private phone companies it licenses under its granted power to regulate inter-state commerce — we don’t see this as the kind of general warrant that the Founders feared. Let Mr. Rubio unpack this if Messrs. Cruz and Rand Paul won’t. Opportunity knocks.

That it takes three senators to pick up the issues that Ron Paul brought to our national debate is something to think about, but there it is. The missing man last night was not Donald Trump, whose absence led to a more discursive, issue-oriented exchange. The missing man is the physician-turned-philosopher who electrified the Republican primary in 2012 by emerging from the study where he spends his hours with the great economists and the authors of the Constitution and talking American principles to the American people.