Donald Trump, the presumptive Republican presidential nominee, is a political outlier not only when it comes to immigration policy, Twitter and marital history, but also when it comes to image.

This isn’t, of course, a coincidence. But it is notable. I am not speaking here of his much discussed hair, or even his somewhat florid skin color. I am speaking of his ties.

And not because, according to CNN, his own-brand ties are made in China and thus seemingly contradict his vow to bring jobs back to America. Or because the ties he actually wears on the stump — the shiny, brightly colored ones with the big knots — are Brioni, made in Italy.

But, rather, because of the sheer fact that he actually wears a tie. Almost all the time. If in the early days of his campaign — while attending, say, the Iowa State Fair — Mr. Trump occasionally lost the neckwear, those days seem increasingly rare.

The pretty much constant presence of the tie has served to highlight another, less discussed but no less pointed gulf between the candidate and those he would call peers: For many politicians, the tie is no longer considered a necessary part of the uniform.

“The president has been wearing a tie less and less,” said Tammy Haddad, a Washington media consultant and former political director of MSNBC. “It is an overt expression of the way this White House has been trying to make politics more human.”

Indeed, all the way back in 2013, the Business Insider website ran a post entitled, “Is President Obama Killing the Necktie Business?”

The president did not wear a tie to dinner with Princes William and Harry during his recent visit to Britain. (They did not wear ties, either.) He did not wear a tie during his news conference about Justice Antonin Scalia’s death in February. (The Internet was not happy.) He did not wear a tie for his opening dinner with President Xi Jinping of China when Mr. Xi arrived in Washington for a state visit last September. (It may have been “informal,” but it had photo ops.)

As it happens, neither did Mr. Xi, Vice President Joseph R. Biden Jr., Secretary of State John Kerry, Secretary of the Treasury Jacob J. Lew or Max Baucus, the ambassador to China. Nor did Jeb Bush when he announced his candidacy for president.

And this is not simply an American development. On Saturday, Sadiq Khan took his oath of office as the new mayor of London in Southwark Cathedral — in a navy suit, white shirt and no tie. The previous week, he and his opponent, Zac Goldsmith, were each pictured on the cover of The London Evening Standard going to vote — in blue suits, white shirts and no ties. In an often contentious campaign, it seemed one of the few tactics both men agreed on. Indeed, the campaign itself was labeled by various Britons as “the most informal in memory.”

But Mr. Khan’s apparent lack of allegiance to the tie does not come close to that of Prime Minister Alexis Tsipras of Greece, who practically made his refusal to wear a tie part of his electoral platform a year ago.

That Mr. Khan and Mr. Tsipras represent left-wing parties may seem significant (Mr. Tsipras being more extreme than Mr. Khan), except that in 2013, Britain’s prime minister, David Cameron, who is a Conservative, decreed, as host of the Group of 8 summit, that the dress code would be “informal,” which translated as tieless. There is a reason one of his nicknames is “dress-down Dave.”

“The tie is an issue that dwells in the minds of candidates, their spouses and their handlers for endless hours,” said Bennet Ratcliff, an international political consultant and founder of Thaw Strategies. “I once had a president spend 15 minutes talking to me about his tie when we could have been discussing the language of a peace accord.”

If the decision on what tie to wear is so complicated, imagine the conversation about not wearing one at all. Such choices are not made by accident, or without an agenda.

Call it Phase 3 of political dress evolution.

Phase 1 was the hatless John F. Kennedy at his inauguration in 1961, signaling to all watchers that a new, breezier generation was in charge. Phase 2 was the lose-the-jacket, roll-up-your-sleeves look, adopted by politicians at the end of the 20th century in multiple in-the-office photo ops, the better to demonstrate their work ethic. And now we are here.

Ties have not disappeared from the political arena, of course. The rules of the House of Representatives demand that men wear a coat and tie on the floor when Congress is in session. (Former Speaker John Boehner was known for rebuking his colleagues if he thought they were showing disrespect to the institution by dressing too casually.) Ditto the Senate. President Obama often wears a tie; so does Mr. Cameron.

World leaders in other hemispheres, of course, have traditionally had a different kind of uniform, one that can involve cultural, often indigenous, garment tropes.

But in the West, there is no question that the tie has become a variable in the political calculation, instead of a constant. Though it is easy to chalk it up to generational change, a more accurate interpretation probably has to do with ideology, opportunism and spin-doctoring. After all, this is a time when social media has meant that the optics of a message — or how it is delivered — are increasingly important. And ubiquitous.

“You can’t overestimate it, but you shouldn’t underestimate it either,” said Steve Hilton, the chief executive of Crowdpac, a site that matches politicians and would-be donors by their priorities, and Mr. Cameron’s former director of strategy. “There is a huge interest now in personal character, and how you dress is an immediate access point for that. It’s a part of an overall message, and a pretty important one.”

This was overtly satirized in a BBC Two show called “The Thick of It” (it inspired the HBO series “Veep”), which ran sporadically in Britain from 2007 to 2012 and featured a communications director called Stewart Pearson. The character was on a mission to update his party, in part by urging elected officials, including his fictional party leader, not to wear ties.

As it happens, Stewart Pearson was inspired by Mr. Hilton, who was widely credited with persuading Mr. Cameron to ditch his tie, the better to sway voters to “take another look at the Conservative party,” as Mr. Hilton said in a phone call from California, where he lives now. It wasn’t the same old, same old — at least when it came to wardrobe. (You have to start somewhere.)

The decision to play hide-and-seek with the tie is “a reflection of the current cultural environment, and an effort to seem like a part of that,” Mr. Ratcliff, the consultant, said, adding: “The leaders are just following the voters. Thank God they haven’t all started wearing black turtlenecks like one unnamed entrepreneur, though it will come to that eventually.”

The Steve Jobs allusion is a reflection of the new economic power structure, one that celebrates the technical entrepreneurial class and the shadow banking sector, both of whose casual style has had a creeping influence on professional dress code, redefining what future success looks like in the popular imagination.

In part, this is how we find ourselves in this weird, inverted sartorial reality, where Mr. Trump has become the exception to the rule because he follows traditional rules. His tie-wearing harks back to the Wall Street uniform of the 1980s, the boom years of the American economy, when it was “morning in America” and Gordon Gekko preached the “greed is good” gospel. The candidate’s mouth says he wants to “make America great again,” and his clothes refer back to the last time many Republicans believed that was actually the case. He is emphatically and consciously not the new-look candidate. He is the old-look candidate.

Thus the tie divide, like many others in this particular election, gets ever wider.