One of the hallmarks of our democratic system is its commitment to the peaceful transition of power. This practice comes with two important, linked corollaries that fall under the umbrella that there can be only one president at a time. The first is that the incoming president, especially in the arena of foreign policy, takes care not to trespass on the prerogatives of the incumbent. The second is that the outgoing president, once departed, remains largely mute, giving his successor space to operate unimpeded by post-presidential back-seat carping.

President-elect Donald Trump must have missed this memo. Not bothering to wait for the constitutionally mandated handover, Trump has inserted himself into policymaking, from bullying U.S. manufacturers to barging into foreign affairs, including shaking up U.S.-China policy and intruding into the Obama administration’s dealings with Israel at the United Nations.

This public tussling is as disturbing as it is unprecedented.

President Richard Nixon, at a news conference a week after being sworn in, was asked whether he would stick with judicial nominations submitted by his predecessor, Lyndon Johnson. Nixon noted that, in conversations as president-elect with Johnson administration officials, he had “scrupulously followed the line that we have one president at a time, and that he must continue to be president until he leaves office on January 20.”

Bill Clinton bookended his presidency with this same point. “During the transition that is now beginning,” he said the day after his election in 1992, “I urge America’s friends and foes alike to recognize, as I do, that America has only one president at a time.”

Eight years later, asked about his plans as he prepared to leave office, Clinton invoked the one-president theme again, saying that he wanted to “find a way to be a useful citizen . . . but to do it in a way that does not get in the way of my successor.”

As president-elect, Barack Obama employed the one-president-at-a-time mantra so frequently — 10 times, by my count — that my colleague Dana Milbank was moved to poke fun at it. In the midst of a financial meltdown, Obama was assertively involved in lobbying for the auto bailout and in shaping the contours of a stimulus package that would be enacted on his watch.

But Obama — who had been criticized during the 2008 campaign for some of his dealings with foreign leaders — steered decidedly clear of weighing in on issues such as the escalating crisis in the Gaza Strip. “We can’t have two administrations running foreign policy at the same time,” he said on Jan. 7. “We simply can’t do it.”

Except Trump can. “At the end of the day, he’s not someone that’s going to sit back and wait,” Sean Spicer, the incoming White House press secretary, told CNN, defending Trump’s involvement with the United Nations vote.

Obama and his aides have, rather gently, made the one-president point. After Trump took a phone call with the president of Taiwan, Obama observed: “Since there’s only one president at a time, my advice to him has been that before he starts having a lot of interactions with foreign governments other than the usual courtesy calls, that he should want to have his full team in place.”

Not only did the president-elect fail to heed the message — he bristled at it. With typical Trumpian gall, he managed to take umbrage at Obama’s conduct during the transition. “Doing my best to disregard the many inflammatory President O statements and roadblocks,” Trump tweeted Wednesday morning. “Thought it was going to be a smooth transition — NOT!”

Trump’s definition of a smooth transition: one that goes entirely his way. So what were these “inflammatory” statements that set Trump off? Just about anything can trigger his wrath. Perhaps it was Obama, at Pearl Harbor with Japanese Prime Minister Shinzo Abe, exhorting Americans to “resist the urge to turn inward” or “to demonize those who are different.” Perhaps it was Obama boasting he would have beaten Trump if allowed to run.

Well, that’s certainly inflammatory. How dare Obama?

Maybe Trump’s disrespectful conduct toward Obama shouldn’t be so surprising. After all, long before the election, Senate Republicans acted as if the Obama presidency had already ended, refusing even to consider his Supreme Court nominee.

In that sense, Trump is merely following an especially ugly, anti-constitutional party line. Imagine how President Trump, in office, would respond to such incursions on his authority — and how President Trump, on his way out, would deal with a successor behaving this churlishly.

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