What do presidential candidates George H.W. Bush, Michael Dukakis, Bill Clinton, Bob Dole, Ross Perot, Al Gore, George W. Bush, John Kerry, Barack Obama, John McCain and Mitt Romney all have in common? On the biggest night each of their political careers, when they — live and on national television without teleprompters or prepared texts — were being scrutinized and judged by up to 80 million of their fellow citizens on their fitness to be president, all of these men agreed to accept and to trust journalist-anchor Jim Lehrer to moderate their presidential debate.

Jim Lehrer, my friend and PBS colleague, departed these earthly precincts in January. I know that France's Charles de Gaulle said that "the cemeteries of the world are full of indispensable men," but with genuine admiration and respect for the just-named 2020 debate moderators — Fox News Sunday anchor Chris Wallace, USA Today Washington Bureau chief Susan Page, C-SPAN political editor Steve Scully and NBC White House correspondent Kristen Welker — Jim Lehrer could truly be irreplaceable.

Let us understand the total pressure of a presidential debate when not only the nominee's campaign but also her/his career is, for 90 minutes live, coast to coast, on the line. It is the only time in these marathon campaigns when we, the voters, have the chance to "interview" the would-be presidents and see how each defends, explains and reacts, as well as what their body language and facial expressions reveal.

Let us also remember what Jim Lehrer taught us by word and example about the duties of the moderator. "The only opinions that matter are those of the candidates," he said, adding, "Nobody cares what the person asking the question thinks." His rule, which he always honored: "You are not here to participate in the debate."

Jim was a superb interviewer and moderator because he always listened to what the person he was questioning actually said. His favorite example of failure to listen was a hypothetical exchange between the host and guest, a U.S. senator: "Should we sell grain to Cuba, Senator?" To which the senator responds: "Yes. We should. But first, we should bomb Havana." The oblivious interviewer's follow-up: "What kind of grain, Senator?"

Jim was occasionally criticized for being too neutral or lacking "attitude" or passion. That is baloney. He thought deeply and cared deeply. Take the enduring issue of guns in America. After graduating from the University of Missouri and being subjected to the military draft, Jim joined the Marine Corps and was commissioned a lieutenant in the infantry serving in Japan and stateside. Jim, a proud Marine, wrote this about guns in America: "Having been expertly trained to use handguns against human beings — particularly all of the body-splitting automatic and semi-automatic weapons — I find it impossible to see them as anything but tools of death and injury. They are not sports equipment. ... Not croquet wickets, not tennis racquets. They are made to cut people in half, to blow holes in their chests, to splash their heads on the pavement. ... I'm grateful to the Marine Corps for having taught me that in a way that stuck forever."

Maybe Charles de Gaulle was right that there are no irreplaceable men. But at the incredibly demanding task of moderating our presidential debates, Jim Lehrer had few equals and no superiors.

He was truly a national treasure, and I miss him.

To find out more about Mark Shields and read his past columns, visit the Creators Syndicate webpage at www.creators.com.