The list of Donald J. Trump’s “worst nightmares” seems to keep growing.

You have Robert Mueller, Stephen Bannon, James Comey, Sean Spicer, Michael Flynn and perhaps Don Trump Jr. standing by to give the Big Man a serious case of heartburn.

Walter Shaub is no slouch in the “worst nightmare” department, though. The former head of the U.S. Office of Government Ethics, took part in a Texas Tribune discussion to talk about government ethics, which in Shaub’s mind is no oxymoron.

He quit his job at OGE out of frustration dealing with the Trump administration.

As the Tribune reports: The last straw for Shaub, who is now with an organization called the Campaign Legal Center, was having to fight for a month to get basic ethics records that did not even turn out to be useful.

In general, Shaub said, Trump’s actions represent a “significant departure” from “ethical norms.” He added that it will be on the next president to repair the damage that’s been done.

“I put up as good of a fight as I could,” said Schaub, who resigned in July.

I continue to believe that the president’s lack of understanding of government has contributed to the ethical morass he has helped create. Trump’s business background simply is not well-suited to adapt to the complexities associated with service in the massive federal government.

Another panelist at the Tribune event, Richard Painter, former ethics lawyer for President George W. Bush, also has been a harsh critic of Trump. According to the Tribune: “People voted for Donald Trump to be a president … not to be a king,” he said. “He’s gotta respect the Constitution or he’s not gonna keep his job.”

There might lie the greatest problem facing Trump as he seeks to shake loose from the crises that are gripping his administration. He doesn’t know — or seemingly want to know — how the Constitution works, how it limits his power and how it sets forth “checks and balances” on presidential overreach.

One such overreach might involve Mueller, the special counsel assigned to investigate the Trump campaign’s alleged ties to the Russian government. As the Tribune reports: “The biggest threat we’re facing is any threat to Bob Mueller being fired,” Shaub said. Should that happen, Shaub said he would take to the streets and that others should too.

Ethics, Mr. President, really do matter. They matter a great deal. Just listen — for once, sir — to these fellows. They know of which they speak.