This post has been updated.
The swiftness and ferocity of the attacks is striking for Cruz, who said as recently as Monday that he didn't want to go to war with Trump. The two men are battling for the top spot in polls in Iowa, which host the first-in-the-nation caucuses Feb. 1. Cruz said Tuesday that Trump was clearly feeling the pressure.
"The Donald seems to be a little bit rattled," Cruz said on "The Howie Carr Show" on Boston's WRKO.
Cruz took direct aim at Trump's competence. "Does a potential commander in chief know what the nuclear triad is, much less is he or she prepared and able to strengthen it and keep this country safe?" Cruz asked on "The Hugh Hewitt Show." In a Republican debate last month, Trump was asked about the nuclear triad and had trouble with the explanation.
And he jabbed Trump on the billionaire's cosmopolitan lifestyle. “I think he may shift in his new rallies to playing New York, New York, because Donald comes from New York and he embodies New York values,” Cruz told Carr.
Cruz had long said he did not want to be engaged in a "cage match" with Trump, even as most of the Republican field grew critical of the billionaire. Still, in recent weeks he had begun tacitly questioning Trump's ability and his electability. His campaign had been gaming out ways Trump could strike against Cruz and how to respond forcefully well before the businessman raised a fresh round of questions over whether Cruz's birthplace affects his eligibility to be president.
For the past week, Trump has said that Cruz should go to court and seek a judgment to resolve the matter of his eligibility for the presidency. A number of Cruz's fellow Republicans —including Iowa Gov. Terry Branstad, Sen. John McCain (Ariz.) and Sen. Rand Paul, the senator from Kentucky who is also a presidential candidate — have also said that Cruz's Canadian birth could be an issue. On Monday, Laurence Tribe, a professor of Cruz's at Harvard Law School, accused Cruz of "constitutional hypocrisy" and said the senator's own legal philosophy would disqualify him from being able to be sworn in as president.
"Laurence Tribe is a liberal Democratic activist. He is a hard-core Hillary Clinton supporter," Cruz told Hewitt on Tuesday. "And suddenly, Hillary Clinton supporters are rushing out to support Donald Trump’s attacks directed at me. And it starts to make you think, gosh, why are Hillary’s strongest supporters trying to prop up Donald Trump?"
Cruz suggested that Trump's increasing jabs at him indicated only that the mogul was "getting nervous."
"Right now, Donald is losing to Hillary by a pretty significant margin in the national polls, and I’m beating Hillary. And so we should expect liberal Democrats to continue echoing Donald’s attacks," Cruz said.
Speaking on NBC News after President Obama's State of the Union address Tuesday, Cruz said that people attacking him is a sign of his strength.
"Four weeks ago, just about every Republican candidate in the field was attacking Donald Trump. Today, just about every Republican candidate is attacking me. I think that suggests maybe something has changed in the race," he said.
Cruz also responded to Obama's implicit criticism of him during the address, when the president called out Cruz's pledge to "carpet bomb" the Islamic State.
"I will apologize to nobody for my commitment to kill the terrorists," Cruz said.