"As a nation we must do more than just echo the phrase Black Lives Matter," he said.
The 250,000 figure is a "myth," the administration says.
The left-leaning survey finds 46 percent who say Trump would kill the mood.
"I don't think it's a good idea to treat anybody differently," said Carson.
"I don't take any criticism from the president seriously," Christie says.
The Lousiana governor dropped his presidential bid on Tuesday.
Fiorina said Obama "doesn't understand the nature of the threat."
Trump asked: "What's our president doing? Is he insane?"