"He’s so nervous," Trump told ABC News' George Stephanopoulos on "Good Morning America." "He’s such a mess."
In response to a clip of the real estate mogul previously saying he was pro-choice, Trump now insisted he is pro-life. Trump also said he supports eminent domain.
Recent polling shows the two outsiders in a tight battle for the lead in the Hawkeye State.
Cruz responded on Monday by saying he wouldn't engage in personal attacks.
"My approach to Donald is the same before -- that I will not engage in insults. I will not engage in personal attacks," he said. "Donald has changed how he has approached me in that now he is insulting me every day and he can do that. That is his prerogative."
Trump also pushed back against Fox News' Megyn Kelly, saying he was still "making a decision" on whether to attend this Thursday's presidential debate. "I don't think she's professional at all. She's very biased against me," Trump said.
“You can probably make that assumption, but I’m thinking about it,” Trump said of attending the debate.
A Fox News spokesperson hit back last night. “Sooner or later Donald Trump, even if he’s president, is going to have to learn that he doesn’t get to pick the journalists -- we’re very surprised he’s willing to show that much fear about being questioned by Megyn Kelly.”
Trump continued to raise question about whether Cruz is a natural born citizen and eligible to hold the presidency. "People realize he probably can't even run for president," he said, citing constitutional scholars, who generally say that Cruz is likely eligible but it is not a matter of settled law.