Criticizing a fellow Republican presidential nominee who was in solitary confinement in a Hanoi prison after being shot down during the Vietnam war. Ridiculing a news correspondent with a physical disability. Taking on a family who lost a son in Iraq. Speaking obliquely about how gun owners might stop a presidential rival.
Only a year ago any one of those actions might have been considered beyond the customary boundaries of American campaign conduct. No longer. Donald Trump has done more than take on the American political establishment, especially that of the Republican Party that has given him its 2016 presidential nomination. He also has shattered the established rules of politics.
That is what insurgents do. In their actions as well as in their words, they assail the assumptions of political and cultural life, winning attention and adherents along the way. The Communist regime in the new Soviet Union, for example, published many of the secret treaties of Russia's onetime allies during the First World War. That was not done by European powers. British Labour Party leader Michael Foot wore a "donkey jacket" at a wartime remembrance at the Cenotaph in London in 1981. That created a furor.
And in the United States, Abraham Lincoln invited blacks, many of them ex-slaves, into the White House. This caused an uproar even in the anti-slavery North.
All of those were deliberate acts, intended to shatter the norms and win notice. Many of Mr. Trump's actions and comments – including perhaps his remarks this week about supporters of the Second Amendment, which protects Americans' right to bear arms – grow out of apparently unplanned rhetorical flights and asides, some the result of impulses more than ideology.
Even so, Mr. Trump has stretched the definition of acceptable political behaviour, in some cases – his discussion of his reproductive organs, for example – causing shock even among his most loyal supporters. And that very stretching has raised these questions:
Are there any barriers left? Are there any sacred cows remaining?
We may discover the answer by 11 p.m. ET on Sept. 26, when the first presidential debate, being held at Hofstra University, Hempstead, N.Y., concludes.
By that hour we will know whether Mr. Trump confronts former secretary of state Hillary Rodham Clinton in person, before a national audience, about her husband's sexual comportment in the White House, and we will know whether Ms. Clinton, in rebuttal, asserts that she fought to preserve her marriage while her rival abandoned his first two wives.
Other candidates, such as Richard Nixon in 1960 and Jimmy Carter in 1980, had the opportunity to raise such questions about their opponents: senator John F. Kennedy of Massachusetts, who had a record of unfaithfulness, and former governor Ronald Reagan of California, who divorced his first wife, Jane Wyman. Neither Mr. Nixon nor Mr. Carter did. Mr. Trump already has raised such questions about former president Bill Clinton, but not on the same stage with Ms. Clinton.
Here are some of the other taboos that remain:
Criticizing the children or siblings of a presidential candidate.
There have been many opportunities to make fun of awkward children (Amy Carter) or troubled brothers (Billy Carter). No one did. Kitty Dukakis, the wife of former Massachusetts governor Michael Dukakis, the 1988 Democratic presidential nominee, suffered from depression. His opponent, former vice-president George H.W. Bush, did not speak of it, ever.
Calling a rival a traitor.
No American presidential candidate has been vulnerable to such a charge. Mr. Trump has criticized Ms. Clinton for her record in the Benghazi violence but has not explicitly described her as a traitor, which has a specific meaning in the U.S. Constitution.
Ridiculing the physical appearance of a rival.
Mr. Trump inched up to that in the primaries when he said of GOP rival Carly Fiorina, ''Look at that face." Abraham Lincoln was ridiculed repeatedly by opponents for being ''homely" but never by Stephen A. Douglas, his principal opponent in the 1860 presidential race. Mary Todd Lincoln, not usually the most supportive of political wives, nonetheless responded, "Mr. Lincoln may not be as handsome a figure, but the people are perhaps not aware that his heart is as large as his arms are long."
And Mr. Lincoln himself, in his 1858 Senate campaign, told a group of political artists, "I cannot see why all you … want a likeness of me unless it is because I am the homeliest man in the State of Illinois."
But there never has been a truly explicit comment about an American candidate's physical appearance that even begins to rival the "Is this a prime minister?" advertisement that prime minister Kim Campbell's 1993 campaign aired that seemed to ridicule Jean Chrétien's Bell's palsy.
Mr. Chrétien's response in Lunenburg, N.S. – "It's true that I speak out of one side of my mouth. I'm not a Tory. I don't speak out of both sides of my mouth" – remains a classic of political rhetoric.
Assailing the few remaining icons.
These include Rosa Parks, Pope John XXIII, Abraham Lincoln, George Washington, Joe DiMaggio, Ray Charles, Shirley Temple, Robert Frost, Carl Sandburg, Dwight Eisenhower, Mark Twain, Jesse Owens, Roger Maris, Walter Cronkite and Gordie Howe. Note that none of these people is still alive.
"The point may be that there are very few taboos left," said L. Sandy Maisel, a political scientist at Colby College in Waterville, Me. "Whether Trump loses because of his attacks on the taboos isn't known. But what is known is that the normal standards of decorum, not just in politics but in life, have changed in a way we couldn't have imagined even a few years ago."
David Shribman