An oxymoron, as Miss White, my high-school English teacher taught us, is a combination of words that is self-contradictory, such as “fresh frozen,” ”found missing” or “guest host.”

But David Cohen, my treasured friend who died this week at 79, was indeed a noble Washington lobbyist who, through a lifetime of dedicated political advocacy, managed to leave our country a more just and more humane place than he had found it.

David Cohen had no illusions about the profession he had chosen and mastered. “Many people disdain all lobbyists. But to me, being a public interest lobbyist is a career you can write home about,” he said.

He worked tirelessly and fought wisely for more than a half-century for civil rights, voting rights and campaign finance reform and against war, more nuclear weapons, poverty and privilege.

Among the more shortsighted posturings of Barack Obama’s winning 2008 presidential campaign was a blanket disdain for all lobbyists. Obama publicly pledged: “They will not run our White House. They will not drown out the views of the American people.”

The Obama team asserted a false moral equivalency between well-compensated efforts to shrink the tax rate of a hedge fund billionaire below that paid by his children’s nanny and citizens’ efforts to stop the United States from invading and occupying Iraq.

The Obama White House could have benefited greatly from the savvy, the experience and the shrewdness of public interest lobbyist David Cohen.

He was not central casting’s idea of a lobbyist. No car, no driver, no power table at the in restaurant. There were no Savile Row suits in his closet, no Gucci loafers on his feet.

He was the picture of “rumpled,” bearing no big-dollar contributions to guarantee direct access to candidates and officeholders.

In a Washington world of mercenaries, David Cohen was truly a missionary.

What made him so effective on Capitol Hill? First, David was always seeking – and creatively finding – common ground. He knew and taught the hundreds of young people he mentored always to seek out and to welcome “converts” to your cause and never to hunt down “heretics” to ostracize or excommunicate.

You must be willing to challenge entrenched power, to respect the views of those with whom you disagree and never to personalize disagreement while understanding that though there sometimes will be conflicts with old allies, there are no permanent enemies.

With his skills at mastering both the subject matter and the personal dynamics of an issue and his exceptional ability to quietly teach and inspire a team for the long struggle, David Cohen, lobbyist, could have made a fortune; he chose instead to make a difference.

He always made time for his friends, for his family, for his colleagues, for those who needed him. He was that too rare liberal who loved real people, as well as humankind.

In his eulogy, Rabbi Ethan Seidel spoke the truth: “David was a mensch. He wasn’t mean. He did not hold grudges. He was an optimist, not just about politics but also about people. He didn’t let his ego get in the way of building coalitions, of finding a path forward. In a town dominated by large personalities and cynicism, David was a striking contrast, with his self-effacing goodness and his ability to be quiet and to listen to whoever was speaking to him.”

I will miss David, and so, too, will millions of people who never knew his name and what he did to give their children a fairer life.

Editor’s note: To find out more about Mark Shields and read his past columns, visit the Creators Syndicate website at www.creators.com.