Witnesses who tuned in to Donald Trump and Barack Obama’s post-election get-together can’t have missed the change in the president-elect’s demeanor and affect.

Quiet and reserved, he seemed almost chastened. Dare I say, humble and deferential to the man whose citizenship he challenged for years leading up to his candidacy.

The real estate tycoon best known for ego, insults and invective seemed almost sensitive and earnest, as well as appropriately respectful toward the president and the rare circumstances in which he found himself.

It was . . . odd.

Was he drugged? Was he too exhausted to rally the showman?

Or was Trump, in fact, feeling the gravity of his accomplishment, the burden of his responsibilities and the nearly sacred aura of his surroundings?

I prefer to think the latter.

As every president learns, there’s a world of difference between running for the office and serving as commander in chief. Suddenly, the lure of the contest and the intoxication of victory are replaced with the overwhelming recognition that you are soon to be the leader of the free world.

From reality show to Oval Office is quite an unexpected career path. If Trump promised his fans they’d win so much that they’d get tired of winning, Trump may be weariest of all. Now what?

But there may have been something else at work Thursday when Trump and Obama appeared as presidential pals at a news conference following their hour-and-a-half-long meeting.

That is, the White House itself brings out the reverent in the irreverent. When you step inside, you become a part of something larger than one individual or 320 million souls. You can feel history breathing in those walls. Walking down grand corridors, heels clicking against marble, you pause for a moment to listen, certain there had been other footfalls behind you.

The business of the free world, the secrets of past administrations and the collected anguish of all previous presidents who, despite prayers and earnest intentions, often found themselves failures — all are collected here. No one measures up to the job.

Now it’s Trump’s turn.

The world waits with a thousand questions, principally: Will he really do what he said he would? Will he put Hillary Clinton in jail, build a wall, ban Muslims and export 11 million immigrants who didn’t follow the rules? Will he really repeal and replace Obamacare?

Will he really seek to muzzle the media, abandon NATO or even consider the real nuclear option? Will he return to being the guy who tweets all night and bullies his enemies?

Or will he give the speech of his lifetime, apologizing for his loose talk and the pain he caused others? I’d be happy to write it for him.

In the meantime, associates, colleagues and employees — former and current — confirm that Trump is a very mixed bag, indeed: An impatient genius who doesn’t read but grasps information quickly; a “screamer,” by his own description in his book “ Trump: Think Like a Billionaire”; a hard-charger who doesn’t sleep and expects 24-7 availability and loyalty from his employees (and his Cabinet?); a no-b.s. artist who sees through others’. Neither a true Republican nor a Democrat, he is by his own account “an army of one.”

Not for long. We hope.

One of his former employees, the optimistically named Louise Sunshine, told The Post that although Trump will be challenged by the demands of the job and the necessity of putting country before ego, “he will do it.” Such faith is about all we have for now, but the image of a humble Trump seated next to Obama was a welcome palate cleanser.

Obviously, Obama is still president and he set the tone — both for the meeting and for the nation to observe. If some Americans haven’t liked the cut of his jib or the contours of his policies, they have reason to feel pride in a leader who modeled class and dignity worthy of his office.

Likewise, Hillary Clinton last week was a shining example of how to accept defeat and surrender nobly to our democratic system’s electoral results, notwithstanding her winning the popular vote.

She, Obama and, yes, even Trump are on their best behavior because, as each has said in her or his own way, the peaceful transfer of power is our solemn duty as citizens and as keepers of civilization’s brightest flame.

Let’s not blow it.

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