Before a crowd of thousands on Tuesday night, former President Barack Obama recalled one of the darkest moments of his presidency, when Dylann S. Roof, a white supremacist, killed nine Black people at a church in South Carolina.

“As president of the United States, my response was not: Who may have influenced this troubled young man to engage in that kind of violence? And now let me go after my political opponents and use that,” he said.

Without mentioning President Trump by name, Mr. Obama delivered an indictment of the president’s approach to politics following the assassination of the right-wing activist Charlie Kirk last week in Utah. In the process, he demonstrated how much Mr. Trump had transformed the American presidency in the years since Mr. Obama walked out of the White House and Mr. Trump first walked in.

In his remarks, Mr. Obama said the job of an American president at a moment like this “is to constantly remind us of the ties that bind us together.”

In his own speeches as president during times of national tragedy, Mr. Obama echoed the efforts of past presidents of both parties, seeking in moments of national shock and grief to reach for the unity that eluded Americans during his eight years in office.