Fighting neck-and-neck in the Iowa polls with Senator Ted Cruz of Texas, Donald J. Trump has gone negative, with a 60-second advertisement assailing Mr. Cruz over a hot-button issue with conservatives in Iowa: immigration.

On Screen

A thumbs-up from Mr. Trump, then right to the attack: “Trump vs Cruz on Immigration,” a full-screen headline promises. A faux television shows Mr. Cruz, in a clip from December, struggling as he tries to parry questions from Bret Baier of Fox News, about an amendment Mr. Cruz offered to legislation that would have overhauled the nation’s immigration laws and created a path to citizenship for people in the country illegally. “Sounded like you wanted the bill to pass,” Mr. Baier says. As Mr. Cruz first stammers his way to a yes, then corrects himself to say he only wanted his amendment to pass, the ad delivers a harsh verdict, branding him “Pro Immigration.”

As Mr. Cruz confusingly tries to dig himself out of a hole with Mr. Baier, another big headline sums up what viewers might be thinking: “What is He Talking About?”

It then musters its evidence: video footage of Mr. Cruz addressing the Senate Judiciary Committee in May 2013. “I want immigration reform to pass,” he declares — adding, after a jump cut, “that allows those who are here illegally to come out of the shadows.” Another verdict by headline: “Pro Amnesty.”