Amid renewed racial tensions after a deadly, and apparently unprovoked, shooting of a black man in Minnesota and another in Louisiana, events capped by an attack on police officers in Dallas during protests, the focus of the presidential race will move to Florida and Pennsylvania on Friday.
In Miami, Donald J. Trump, the presumptive Republican nominee, will give a speech — the topic had not been revealed by Thursday evening — and he will appear with Gov. Chris Christie of New Jersey, one of his vice-presidential prospects.
It will be his first trip in a month to a battleground state, one with a large Hispanic population. And he has tried with deeply mixed levels of success to sustain a case against Hillary Clinton, the presumptive Democratic nominee, over admonitions from the F.B.I. director, James B. Comey, about her “extremely careless” use of a private email server.
Mrs. Clinton, who is seeking to reassemble President Obama’s winning coalition amid the rise of the Black Lives Matter movement, has made criminal justice overhaul a hallmark of her campaign.
The graphic and disturbing Facebook Live video filmed by Mr. Castile’s girlfriend immediately after the shooting was among the most visceral portrayals of a police interaction in modern memory. It played on cable news throughout Thursday.
Mr. Trump, whose poll numbers with African-American voters are low, has not addressed Mr. Castile’s and Mr. Sterling’s shooting deaths. And the landscape changed instantly Thursday night from videos of shootings of black men by the police that have become painfully familiar, to the horror of a major mass shooting of officers. How Mr. Trump handles the events of the last three days will say a great deal about the remaining four months of the 2016 campaign.