A voter asked Ted Cruz to name his greatest personal failing during a Republican town hall event. CNN moderator Anderson Cooper decided to volunteer his own: "My greatest failing is sometimes I don't follow up."

Not that night. Cooper was cordial but relentless as he pressed the candidates to explain their positions, to stay on topic and to stop acting like children.

"But, sir, with all due respect, that's the argument of 5-year-old," Cooper told Donald Trump when the GOP front-runner protested that he "didn't start" the shameful feud with Cruz about both candidates' wives.

Butting in required Cooper to abandon conventional principles. It was also hard because of the sheer volume of Trump's misleading claims and because the billionaire hates to be interrupted. He's always ready to take back control of the conversation with a forceful "excuse me."

In fact, Trump wound up dropping 18 "excuse me’s" on Cooper in one hour.

It wasn't just real-time fact-checking that made Cooper's session with Trump so good. He also showed sensitivity for subtle inconsistencies and refused to let them slide. For example: a voter asked Trump to list what he considers the top three functions of the federal government; the candidate named national security, education and health care. That's a fine priority list — except that Trump wants to repeal Obamacare — which increased government's role in health care — and has railed against the federal government's role in education. What was he talking about?

COOPER: Aren't you against the federal government's involvement in education? Don't you want it to dissolve to states?

TRUMP: I want it to go to states, yes. Absolutely. I want — right now ...

COOPER: So that's not part of what the federal government's ...

(CROSSTALK)

TRUMP: The federal government, but the concept of the country is the concept that we have to have education within the country, and we have to get rid of Common Core, and it should be brought to the state level.

COOPER: And federal health care run by the federal government?

TRUMP: Health care — we need health care for our people. We need a good — Obamacare is a disaster. It's proven to be ...

COOPER: But is that something the federal government should be doing?

TRUMP: The government can lead it, but it should be privately done.

That was first-rate nonsense: "The concept of the country is the concept that we have to have education within the country?" It should have been obvious to anyone paying attention that Trump's policy prescriptions are pretty vapid.

Notice the word should. Neither Cooper nor news media can foist conclusions on voters, but this was a terrific example of a journalist doing his job — asking tough informative follow-up questions.

Cooper was equally sharp with Cruz. One of the best exchanges centered on the Texas senator's statement that "we need to empower law enforcement to patrol and secure Muslim neighborhoods before they become radicalized." The remark was a classic case of tough talk that a candidate would rather not explain in any detail. So Cooper asked Cruz what he meant four times and repeatedly pointed out that the closest analogue for his proposal — a surveillance program New York police piloted but ultimately abandoned —simply didn‘t work, according to the New York City police chief.

Cruz insisted, against all evidence, that "this was a successful program," and Cooper didn't let him get away with it.

In another moment, when Cruz launched into a complaint about how much "free media" cable news networks have given to Trump, Cooper responded: "Well, I got to say, we've asked you for interviews pretty much every day and you've declined every offer on my program."

Educator Reporter Al Bruce writes a weekly column appearing Friday in The Evening Tribune.