Crises that arise during presidential campaigns often define the candidates.
Will this horrific week prove to be the crucible of the current campaign?
Violence has shuddered through America since Tuesday: First, two controversial shootings by police of African-American men, captured on cameras and spread on social media; then the assassination of at least five Dallas police officers and the wounding of others by snipers after a peaceful march protesting the earlier deaths.
At the very minimum, the bickering between Donald Trump and Hillary Clinton has been temporarily overshadowed, much as it was less than four weeks ago when a single assailant killed 49 people and wounded more than 50 others in an Orlando, Fla., nightclub.
That pause proved temporary and, for all its horror, had little effect on the presidential race.
But in past decades, dramatic disorder has had a political impact. The convulsions of protests and violence in 1968 helped swing the presidential election that year _ one of the closest in history _ to the law-and-order candidate, Republican Richard Nixon.
The effect of the latest outbreak may be fully determined only when more specifics are known about the Dallas attack, particularly the identities and motivations of the snipers.
Already, however, the attack has served as a reminder of how events outside the campaign can overwhelm the carefully plotted plans and strategies of the candidates. For a time, at least, the question of whom Trump will pick as a running mate and the details of Clinton's handling of classified information in her emails while secretary of State seem unlikely to attract much attention.
Whether the effect goes deeper and persists also will depend on how the public _ and the candidates _ frame the week's deadly events with their fraught elements of racial tension and maintenance of public order.
Both candidates will have opportunities to begin shaping their responses _ and the public's view of events _ in coming days. Clinton was already scheduled to give a speech about the relationship between police and minority communities at a historically black church in Philadelphia. Trump had planned to campaign in Florida, making his first appearance since clinching the GOP nomination, with Sen. Marco Rubio, one of his defeated rivals, but postponed the event.
Trump has built much of his campaign around the idea that America is no longer "safe." He couples his denunciations of illegal immigration with claims that American cities are places of danger where his audiences _ mostly older, white and nonurban _ would rightly fear to walk.
He could benefit again if voters see this week's killings as part of a general picture of national chaos _ a crisis of authority requiring a tough response, as many did in 1968. That reaction could prove particularly powerful if the Dallas attack proves related to terrorism.
In a statement released early Friday, Trump said it was necessary to "restore law and order" and "the confidence of our people to be safe and secure in their homes and on the street."
"Our nation has become too divided," said Trump, who has been criticized for divisive rhetoric throughout the campaign. "Crime is harming too many citizens. Racial tensions have gotten worse, not better."
But it is also possible that voters craving a sense of security will seek out a more deliberate, less unpredictable leader in Clinton. She also has made gun violence a central tenet of her campaign for more than a year, giving her an authenticity on that element of the conversation that Trump lacks.
A search for steadiness in a troubled time boosted then-Sen. Barack Obama and hurt Sen. John McCain in 2008 when the Republican's haphazard reaction to the nation's economic collapse helped define him as a riskier bet for voters. The calmer response by Obama, still a relatively unknown figure nationally that fall, helped voters see him as capable in the face of calamity.
In a tweet sent Friday morning, Clinton focused on those slain in Dallas.
"I mourn for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them," she wrote.
A poll published Thursday before the paroxysm in Dallas detailed Clinton's and Trump's strengths in the minds of American voters. (Clinton led Trump in the poll, by the nonpartisan Pew Research Center, 51 percent-42 percent.)
Asked which candidate possessed the best judgment in a crisis, 53 percent of voters said Clinton, to 36 percent for Trump. But Trump rose when voters were asked which was better at defending against a terror attack, with 48 percent for the Republican to 43 percent for Clinton. Voters were split over which candidate was better at gun policy.
That suggests that if voters frame recent events as a domestic crisis, Clinton may benefit. If the Dallas killings are seen as terrorism, Trump may benefit.
But voters' views of Trump and Clinton are so negative _ and supporters of both are already so dug in _ that even their responses to a week such as this may not change voters' perceptions.
Another problem facing the presidential candidates as they seek to serve as unifying figures in the aftermath of the week's violence: Its elemental parts inspire great division.
Segments in the country see the shootings of African-American men as a demonstration of a biased criminal justice system. Others give the benefit of the doubt to police officers who are integral protectors of society.
The division has played out with strong support by young and minority Americans for the Black Lives Matter movement _ and far less support for it among older, white Americans, a divide that echoes the Clinton-Trump split among voters.
President Obama has sought to bridge that gap. Before the Dallas violence erupted, he spoke in Poland, where he had traveled for a NATO summit, of the shootings in Louisiana and Minnesota. He suggested both African-American victims and diligent police officers should be embraced.
"This is not just a black issue. It's not just a Hispanic issue. This is an American issue that we should all care about. All fair-minded people should be concerned," he said of the shootings by police.
In the next sentence, he praised police officers with words that would echo painfully just hours later.
"They've got a dangerous job. It is a tough job," he said. "And as I've said before, they have a right to go home to their families, just like anybody else on the job."
After Thursday night, at least five of them will never again go home to their families in Dallas, a fate shared by men in Baton Rouge, La., and Falcon Heights, Minn. The challenge ahead for the presidential candidates _ and a steep one given the often dismal contours of this campaign _ is to persuade the nation that it can mourn all seven, simultaneously, with something approaching unity.
