Mark Penn, the morally flexible pollster to Bill Clinton now advising President Trump on how to beat impeachment, told him to “govern” and focus on “substance” without reacting to every development.

Penn might as well have told Trump to go without Diet Coke and a hair dryer.

Trump’s visit to London for this week’s NATO summit would have been a perfect opportunity to showcase his governing substance. But immediately upon landing, he tweeted that his in-flight reading had been the Republican rebuttal of “the Impeachment Hoax,” saying the “Radical Left has NO CASE” and asking: “Can we go to the Supreme Court to stop?”

Apparently there was no copy of the Constitution in the seat-back compartment.

Trump continued tweeting impeachment grievances, and, in his first public meeting Tuesday, with the NATO secretary general, he pronounced Democrats disloyal for carrying on with impeachment while he traveled.

“It’s very unpatriotic of the Democrats,” he said. He later added: “They are hurting our country very badly.”

This from a man who challenged the loyalties of a Purple Heart recipient in his own White House, ordered the military to not punish accused war criminals, attacked the heroism of John McCain and the sacrifice of Gold Star parents, spurned veterans events and memorials, and used tombstones of the fallen as a political backdrop. But then, Trump doesn’t distinguish between love of country and love of him; he once suggested those who didn’t applaud his State of the Union speech may have committed treason.

French President Emmanuel Macron, in the run-up to the NATO conference, warned that we are witnessing the “brain death” of the military alliance since Trump’s unannounced pullout from Syria and Turkey’s invasion. But the brain causing the most concern in London this week is the one belonging to the impeachment-distracted American.

Trump twice informed The Post’s Phil Rucker on Tuesday that he had read something in “your paper” or “your newspaper” — forgetting he supposedly ended the White House’s subscription to The Post. He said that until his presidency, the United States only “very rarely” won a case before the World Trade Organization; the United States actually won more than 90 percent of cases it brought. He said he predicted the Brexit outcome “the day before” the vote; it was the day after. He said, “I don’t know Prince Andrew”; the photos say otherwise. He gave himself credit for increasing NATO member military spending by $130 billion “annually”; that’s actually an increase over several years, from before he took office. And he said the “common foe” of Europe — Russia, presumably — “may not be a foe.”

Trump gave the Europeans a familiar show of ego (“I do pretty well with Twitter,” “I’ve won a lot of elections for a lot of people”) and carelessness (his remarks about Iran required a clarification and his thoughts about China sent stocks diving). It fell to others to play the role of statesman previously played by American presidents. Macron chided Trump over Syria, where Turkey is “fighting against those who fought with us shoulder to shoulder against ISIS,” and he shut down Trump’s attempt to blame the Obama administration for Turkey’s purchase of Russian weapons. When Trump joked about sending Macron “some nice ISIS fighters,” Macron told him “let’s be serious” and reminded him that the fight against the terrorists is “not yet done.”

Though his feet were on foreign soil, Trump’s head remained in Washington, where Democrats on Tuesday afternoon released their compilation of the evidence against Trump, outlining a scheme that “undermined our national security in favor of two politically motivated investigations that would help his presidential reelection campaign.” They wrote that despite “an unprecedented campaign of obstruction,” they “uncovered significant misconduct.”

Trump, responding to questions from Rucker and Steve Holland of Reuters, repeatedly diverted from international affairs to impeachment, as captive foreign leaders sat at his side.

NATO’s secretary general, Jens Stoltenberg, got an earful about the “impeachment hoax,” “witch hunt” and “perfect call.” He listened as Trump declared a censure resolution “unacceptable” and falsely claimed the Ukrainian president had just said “President Trump did nothing wrong.”

Soon it was Canadian Prime Minister Justin Trudeau’s turn to listen to more of the same: “Where’s Hunter?” “A total fix.” Asked what Penn had advised him, Trump rambled on about his “biggest fundraising month” and “best poll numbers.” He called Intelligence Committee Chairman Adam Schiff (D-Calif.) “a maniac,” a “deranged human being” and “a very sick man.” Adding in House Speaker Nancy Pelosi (D-Calif.), Trump said: “These people are deranged, okay?”

It’s all Penn’s fault for telling Trump to stick with governing and substance. Epithets are his substance, and disinformation his way of governing.

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