If Barack Obama had any qualms about meddling in the affairs of other countries, he might have long ago ceased to be U.S. president. It comes with the vast, troubled territory.

U.S. president makes a last official visit to the U.K. with a special stop at Windsor Castle 1:34

Now, near the end of his presidency, it has landed him in the U.K. in the midst of the country's bitter debate on whether to stay in the European Union, and that has earned him a severe, farewell British bollocking.

"Obama's Brexit overreach is typical of his arrogance," was one headline in the The Spectator weekly.

"There is a longstanding international understanding that world leaders don't visit during election campaigns," writes Tim Montgomerie, a self-described Brexiteer.

"But such conventions were obviously designed for lesser mortals."

Obama comes just in time to fete the Queen for her 90th birthday over lunch today and to touch nerves at a hypersensitive moment in British politics.

U.S. President Barack Obama, seen here with Queen Elizabeth when he was in London in 2011, will sit down for lunch with her today to celebrate her 90th birthday. (Larry Downing/Reuters)

Even the Queen was the subject of angry headlines last month when it was suggested she backed Brexit. Buckingham Palace denies it.

But Obama's timing, just weeks before the June 23rd referendum, is just one, big, sticking point for Brexiteers to be added on top of the message Obama carries on what is likely his last visit here as president.

Talking as a 'friend'

In a letter published in the Daily Telegraph today, Obama says "with the candour of a friend" that the outcome of the vote is a matter of "deep interest" to the U.S.

He acknowledges there's been controversy over the timing of his visit, then lays out his argument for sticking his nose in.

"I confess: I do want to wish Her Majesty a happy birthday in person," he writes.

But, he continues....

"The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic.

"So the US and the world need your outsized influence to continue - including within Europe."

He does preface that argument with an acknowledgement that ultimately the question is "a matter for British voters to decide for yourselves."

It is the standard line deployed when the heads of the IMF, the World Bank and NATO recently weighed in against Brexit, too.

So where does respectful advice end and meddling begin?

That depends on the level of desperation and the magnitude of the threat to the status quo. That, in turn, determines how loudly to meddle.

U.S. presidents might make a habit of it behind the scenes, but they do go public when necessary. Witness, for example, the Scottish referendum, when Obama weighed in publicly against independence.

And that privilege seems to apply equally to the president as it does down to the U.S. ambassador to Beirut, a position that apparently allows its holder the same right to public commentary on Lebanon's internal politics as, say, Lebanese Druze chieftain Walid Jumblatt.

'Special relationship'

The U.K., of course, is one side of a "special relationship" with the U.S.

And Prime Minister David Cameron, who is heading the Remain campaign, could apparently use a boost from the only world leader who calls him "bro." Cameron argues Britain is safer, more prosperous and more powerful in a reformed EU.

British Prime Minister David Cameron has said he would campaign with all his 'heart and soul' for Britain to stay in the European Union. (Yves Herman/Reuters)

Obama's endorsement and popularity could help.

Whether it actually does is another question. But just hours before the Scottish referendum vote, Obama did weigh in, not only down the line across the Atlantic — but on Twitter.

"The UK is an extraordinary partner for America and a force for good in an unstable world. I hope it remains strong, robust and united," he wrote. Surely, it was because it was in the U.S.'s interests, too.

It is no wonder big players are increasingly wading in publicly, hoping to wield influence directly with people in a world when the EU could cease to exist and Donald Trump could become U.S. president — because it affects them, too. And because they can.

But where in this over-connected world does the meddling red line now stand?

You can try asking the Pope.

More praise than put-downs

He was just accused of meddling in politics — an arena most would agree should lie beyond his purview as a religious leader — twice last week.

One accusation came after he visited stranded refugees on Lesbos, Greece, an act some saw as his silent commentary on Europe's controversial decision to send some of them back to Turkey.

Pope Francis welcomes a group of Syrian refugees after landing at Ciampino airport in Rome following a visit to a refugee camp on the Greek island of Lesbos on April 16, 2016. (Filippo Monteforte/Reuters)

The other instance was for meeting U.S. Democratic leadership contender Bernie Sanders in the lobby of his residence at the Vatican during what is a hotly contested run-up to the next U.S. presidential election.

This, of course, is the same Pope who seemed to get more praise than put-downs when he took on Trump earlier this year, calling his idea to build a wall to keep migrants out of the U.S. "not Christian." Trump dismissed the comment as "disgraceful."

On the apparently warm — but brief — Sanders meeting, the pontiff defended himself directly.

"This is called good manners," he said.

"If anyone thinks that greeting someone is getting involved in politics, I recommend that he look for a psychiatrist."

Rational nuance

The U.K.'s chief Brexiteer, London Mayor Boris Johnson, can be bombastically eloquent on many things. On Obama's anticipated intervention, he seems to border on distraught.

He called it "a piece of outrageous and exorbitant hypocrisy" and "nakedly hypocritical."

London Mayor Boris Johnson says he is fine with U.S. President Barack Obama having a say in the debate over Britian staying or leaving the EU, but he has a problem with the president’s argument. (Andrew Yates/Reuters)

There is, however, a rational nuance in Johnson's statements on this.

There is meddling, he seems to say, and then there's meddling.

Unlike many in his camp, Johnson says he is fine with Obama having a say in the debate but he has a problem with the president's argument.

The U.S., says Johnson, would "never dream" of giving up its sovereignty to a multinational organization resembling anything like the EU. So why would it publicly ask its closest ally to do so?

Now that's using meddling to your advantage. Or at least trying.