Another candidate, former Vice President Joseph R. Biden Jr., was scheduled to deliver remarks on Wednesday in Burlington, Iowa. “Trump offers no moral leadership; no interest in unifying the nation, no evidence the presidency has awakened his conscience in the least,” Mr. Biden is expected to say, linking the president’s rhetoric to the El Paso shooting and a shooting last fall at a Pittsburgh synagogue. “Instead we have a president with a toxic tongue who has publicly and unapologetically embraced a political strategy of hate, racism, and division.”
And former Representative Beto O’Rourke has been in El Paso, his hometown, since the shooting and is likely to make remarks during Mr. Trump’s visit to that city late Wednesday afternoon.
Mr. Booker’s choice of venue in South Carolina is laden with symbolism: the predominantly black congregation in downtown Charleston was attacked in 2015 by a white supremacist who killed nine people and has since been sentenced to die for his crimes. After that massacre, the church, known as Mother Emanuel, became the site of one of the most memorable moments of former President Barack Obama’s time in office.
At a funeral for the victims, Mr. Obama broke into a rendition of “Amazing Grace,” and called both for the removal of the Confederate flag from the South Carolina capitol and for a remedy to the “mayhem that gun violence inflicts upon this nation.”
Mr. Booker will not be delivering his remarks from the same presidential pulpit, and there was some political risk involved in planning his speech from a place so laden in history. Some Democrats believe he has suffered in the 2020 campaign, fairly or not, from voters’ instinct to measure him in their minds against the example of Mr. Obama.
Yet the present moment of turbulence and trauma has quickly developed into a vitally important one for Mr. Booker, who after months of toiling away in relative obscurity has begun to show signs of breaking through in the presidential race. He had a standout performance in the second round of Democratic debates last week, besting Mr. Biden in a series of exchanges on race and criminal justice and displaying for a national audience the kind of sunny pugilism that has made him a force in New Jersey and in the Senate.
He remains a decided underdog. A Quinnipiac University poll published on Tuesday found that 2 percent of Democratic voters favored him for the nomination, compared with 32 percent for Mr. Biden, 21 percent for Senator Elizabeth Warren of Massachusetts and 14 percent for Senator Bernie Sanders of a Vermont. At no point so far has Mr. Booker managed to make himself a major voice in the Democratic presidential field’s raging debates about the economy and the role government should play in addressing widespread social inequality.