American presidential campaigns are shaped by great mainsprings of history, by partisan trends, by candidate choices – and, to a startling degree, by events that no one could have predicted. But one prediction is risk-free: The spate of controversial police shootings of black Americans, and then Thursday's shooting of police in an ugly Dallas confrontation, will shape the 2016 presidential campaign in the United States.
By the time the morning papers were digested Friday, both Hillary Rodham Clinton and Donald J. Trump had cancelled important campaign events – an appearance by the former Secretary of State with Joseph Biden Jr. in the vice-president's hometown of Scranton, Pa., a city that is a sturdy symbol of working-class angst, and one by the Manhattan businessman in a Florida venue with a large number of Hispanics, a voting group that has been chilly to Mr. Trump. Both appearances were in swing states the two campaigns consider vital to building electoral college majorities in the fall.
But even as the day's political events were cancelled, there was growing concern in the campaigns and in the country about these incidents. "We have a kind of toxic mix in the perceptions that exist in the minds of police officers and in the minds of African American citizens," said David A. Harris, the University of Pittsburgh law school professor considered the leading authority on the conduct of American police forces.
The shootings – for it is impossible to separate the incidents in Minnesota and Louisiana, where black men were shot this week, and the shooting in Dallas, where police officers were targeted – present unusually difficult challenges for both candidates.
By early morning Friday, Ms. Clinton said in a Tweet that she mourned "for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them."
Mr. Trump, who projects toughness and an affinity for police officers, assailed the Dallas attacks. "We must restore law and order," he said in a statement. "We must restore the confidence of our people to be safe and secure in their homes and on the street."
Though Mr. Trump had earlier spoken with concern over the "senseless, tragic" deaths of black motorists, he swiftly was facing charges Friday, from the Reverend Jesse Jackson, himself a former two-time Democratic presidential candidate, that he and some of his supporters were contributing to what the civil-rights leader called "a mean-spirited division in the country."
For her part, Ms. Clinton has embraced elements of the Back Lives Matter movement but shares the queasiness that liberals have felt for almost a half century over law-and-order issues.
All this is complicated further by the respect that presidential candidates, who have around-the-clock Secret Service protection supplemented by local police forces, come to acquire during bruising political campaigns.
They see police officers multiple times every day and, in time-honoured but genuine gestures, make special efforts to thank them for their service, their diligence and their courage. In Ms. Clinton's case, this exposure to police officers includes four years in presidential campaigns – two each for her husband and for her – and eight more years in the White House as First Lady, plus a dozen years in the governor's mansion in Arkansas while her husband was the state's chief executive.
Mr. Trump has long been a vocal supporter of police officers, and as long as five months ago tweeted: "The police in our country do not get respect. Our law enforcement officers deserve our appreciation for the incredible job they do."
Violent acts and episodes involving police have often shaped American presidential elections. The decisive response to the Boston police strike of 1919 catapulted governor Calvin Coolidge of Massachusetts into prominence and into the Republican vice-presidential nomination in 1920; when his running mate, president Warren G. Harding died in office in 1923, Mr. Coolidge ascended to the presidency.
An earlier shooting in Dallas, the 1963 assassination of president John F. Kennedy, put Lyndon B. Johnson in the White House, and the 36th president's brilliant transition in the 11 months before the 1964 election contributed to his landslide triumph over senator Barry Goldwater of Arizona. The assassinations of civil-rights leader Dr. Martin Luther King Jr. and senator Robert F. Kennedy of New York, combined with urban violence and rioting at the Democratic National Convention in Chicago, were important elements in Richard Nixon's victory in the 1968 election.
Indeed, it was in 1968 that the phrase "law-and-order" won its earliest prominence in American presidential elections.
Mr. Nixon used the term with ease, and it won him adherents among voters impatient with racial upheaval and with student violence on campus. All the presidential candidates in 1968, with the exception of Democratic senator Eugene McCarthy of Minnesota, spoke openly about violence in American life, but the phrase "law and order" did not fit comfortably into the Democratic lexicon.
"The very term 'law and order' enraged millions of good, self-styled 'liberals' across the country," the election chronicler Theodore H. White wrote in his bestseller, The Making of the President 1968. Mr. White explained that these voters saw the phrase as "code word for racism," though he pointed out that blacks in Chicago and New York asked for more police officers to battle crime in their neighbourhoods.
Some five decades later, the situation is even more complex. "African Americans are feeling endangered in their encounters with the police, and the police officers are in fear of the presence of guns," said Mr. Harris, the University of Pittsburgh law professor. "When you have fear working in these tense situations, things go terribly wrong in a hurry. The results can be catastrophic for everyone involved."
David Shribman, executive editor of the Pittsburgh Post-Gazette, won a Pulitzer Prize for coverage of U.S. politics.