Supporters of remaining in the union were also scathing in their criticism of Mr. Farage on Sunday.

Scotland’s first minister, Nicola Sturgeon, called for the poster to be withdrawn and described it as “vile and racist.” George Osborne, the British Treasury chief, said it had “echoes of literature used in the 1930s.”

Mr. Farage rejected the criticism, telling the ITV television network that, far from stoking hatred, he had “been a victim of it.” He described the poster as a reflection of “the truth” and added that Mr. Gove’s campaign had also released “very strong posters.”

But Mr. Farage also said the death of Ms. Cox might have had a negative effect on the chances of those who support a withdrawal, or “Brexit.” “We did have momentum until this terrible tragedy,” he said.

“When you are taking on the establishment, you need to have momentum,” Mr. Farage said. “I don’t know what’s going to happen over the course of the next three or four days.”