Disagreements on race relations and guns have become firmly entrenched in the politics of the world's most powerful nation.
Black voters are more likely to see race relations as a bigger problem, and they overwhelming support the presumptive Democratic presidential nominee. While Clinton postponed a rally she had planned for Friday, she was still expected to attend a convention for the African Methodist Episcopal Church, a predominately black denomination.
Many Republicans are unwilling to give any ground to tighten the nation's gun laws. Trump constantly uses the issue to excite the party's base, inaccurately accusing Clinton of proposing to eliminate the Second Amendment right to bear arms.
More than a third of Americans said they worried "a great deal" about race relations in the United States, according to a Gallup poll in April, which was higher than at any time since the polling firm first asked the question in 2001.
That poll showed that concern about race relations over the past two years increased among Republicans and Democrats, conservatives and liberals, and blacks and whites. But the gap between the groups in some cases has widened: 53 percent of blacks said they were worried, compared to 27 percent of whites. That was up from the gap of 31 percent to 14 percent between blacks and whites in the 2012-2014 combined polls.
On gun laws, about 55 percent of non-white Americans support tighter control, a higher proportion that the rest of the country, according to a CNN poll in October. There's also a gender gap on the issue: Women were split about evenly, 49 percent to 48 percent, on stricter laws, while men opposed stricter laws 56 percent to 42 percent.
In the 2016 election, while black voters side with Clinton by a large margin, whites more narrowly prefer Trump, according to a Quinnipiac University national poll conducted in June.
The disputes help explain why it's been difficult to make progress on these issues for President Barack Obama.
In Dallas, Police Chief David Brown said Friday that one suspect in the attack said he was upset over the recent police shootings of black men and wanted to kill white people, according to the Associated Press.
Recent deaths in Minnesota and Louisiana-parts of which were captured on widely viewed videos-were the latest in a string of high-profile incidents in which white police officers have killed black men.
"The senseless, tragic deaths of two motorists in Louisiana and Minnesota reminds us how much more needs to be done," Trump said in his statement about Dallas.
In 2015, riots erupted in Ferguson, Missouri, after 18-year old Michael Brown was killed by Darren Wilson, a 28-year-old white officer. A subsequent investigation by the Department of Justice found that the police department there had routinely violated the civil rights of black residents.
Protests also erupted in Baltimore after 25-year-old Freddie Gray died while being transported in a police van in Baltimore. That incident followed the 2012 killing of Trayvon Martin, who was shot by a mixed-race Hispanic man named George Zimmerman, and the death of Staten Island man Eric Garner, who died after being put in a chokehold by police. Video of that incident also went viral on the internet.
Obama caused controversy early in his presidency when he said a Massachusetts officer "acted stupidly" when he arrested Harvard professor Henry Louis Gates, Jr., who is black, in his Cambridge home. Obama has been criticized by some activists for not being more vocal about policing issues, particularly as the first black president of the U.S.
In recent years, Obama has met with leaders of the Black Lives Matter movement and spoken in detail about how he empathizes with blacks who are profiled. He said that if he had a son, he would look like Martin. Obama has also spoken about instances where he had been racially profiled before entering the public eye.
The Obama administration created a task force in response to the controversies, and the president has urged local police forces to implement new guidelines drafted by the Justice Department to improve community relations. He's also advocated the adoption of body cameras by police officers, and encouraged cities to be more open with data about police and race.
The 11-member task force created by Obama after the high-profile police killings in New York and Ferguson recommended steps it said could help police departments repair frayed relationships with minority communities.
Among the 59 recommendations released by the panel were guidelines suggesting restrictions on the kinds of physical force used against "vulnerable populations" such as children and disabled people; hiring diverse police forces; and using body cameras to document interactions with police. The report also recommends limiting the use of riot gear, which police in Ferguson donned as they confronted protesters, and suggested new rules-subsequently adopted by the Obama administration-governing the transfer of certain military equipment to local police departments.
The administration also recommended departments collect and publicize demographic information about people they stop or arrest.
The president has asked Congress or a three-year, $263 million community policing package that includes $75 million, along with matching funds from local governments, to supply as many as 50,000 body-worn cameras for officers. Last year, the Justice Department announced it was awarding $23 million in funding to law enforcement agencies across 32 states as a pilot program to examine the impact of body cameras.