"I don't know," Trump said. "Honestly, we don't know. Who the hell knows."

Cruz was Trump's No. 1 target during the 65-minute event, revealing just how much of a threat the Republican senator from Texas has become to the front-runner. Before the rally started, Bruce Springsteen's "Born in the U.S.A." blared, a new addition to Trump's playlist.

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"So, Cruz is a problem," Trump said, beginning an attack that lasted about seven minutes. "And here's the problem: It's called uncertainty. It's called you just don't know."

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Cruz has repeatedly said there is no question that he is eligible for the presidency, saying this weekend that "the Constitution and federal law are clear that the child of a U.S. citizen born abroad is a natural-born citizen." Cruz's campaign has yet to respond to Trump's latest comments.

But Trump said Sunday that "this is not a settled matter" and that he's not the only one raising questions. He said if Cruz becomes the Republican nominee, the Democrats could challenge his eligibility in lawsuits that could drag on for years.

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"Does anyone know more about litigation than Trump?" Trump said of himself. "Okay? I know a lot. I'm like a PhD in litigation."

Trump compared Cruz running for president with this lingering question about Democrat Hillary Clinton running despite lingering questions about her use of a private email account during her time as secretary of state. Later Trump also compared the situation to a fighter being disqualified for not meeting the weight class.

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"So she's got the cloud hanging over her head, but Ted Cruz has a real cloud hanging over his head," Trump said. "So the question is: Is Ted Cruz, is he a natural-born citizen?"

The crowd again shouted, "No!"

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"I just heard this: He was a citizen of Canada for a long time," Trump said, referring to Cruz having citizenship in the United States and Canada until recently. "He was a citizen of the United States, I believe, and Canada simultaneously. How do you, how -- what's going on here? So, he's got to straighten these things out."

Trump questioned why Cruz didn't revoke his Canadian citizenship years ago, especially when he became a U.S. senator.

"Does he get a pass from that?" Trump asked. The crowd again answered, "No!"

There have been other presidential candidates who were not born in the traditional United States, but Trump says their cases are different. An example he gave: Sen. John McCain (Ariz.), the 2008 Republican nominee, was born to two U.S. citizens on a military base in Panama.

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"I understand that -- do we understand?" Trump said, as the crowd cheered. "That's good: Parents are military, they're on a base. What are you going to do?"

Trump called Cruz "a good guy," but took a series of shots at him, saying that the senator is controlled by a small number of wealthy donors and changed his position on ethanol to win over Iowa voters. He also hit Cruz for being "weak on illegal immigration" and "totally in favor of amnesty" for illegal immigrants already in the country. He accused Cruz of borrowing his immigration ideas, including his call to build a wall along the U.S-Mexico border.

"By the way, who's going to pay for the wall?" Trump asked. The crowd shouted back, "Mexico!"

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Cruz wasn't the only topic Trump covered: He urged the crowd to buy the most recent edition of Time magazine and read the cover story about him, which he described as "fair." He slammed television reporters at the rally for not fully showing his crowd. He professed his love of protesters, who attract the attention of the cameras away from him and to the crowd. He bragged about having spent $700 million doing business in Nevada and making a "fortune" in Atlantic City before it went under. He boasted of not having a super PAC like other candidates but promised to still be "greedy for the United States of America." He paused when a woman in the crowd collapsed and then marveled: "She's surrounded by doctors, no thanks to Obamacare."

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And amid this circus-like atmosphere, Trump said he likes being compared to P.T. Barnum, the showman who founded the Barnum & Bailey Circus.

"We need a P.T. Barnum," Trump said, citing a comparison that had been made Sunday morning on NBC's "Meet the Press." "I'll tell you what: We need energy in this country of ours because we're losing our spirit. No, we are losing our spirit. We are losing our spirit. We need somebody that's going to be a cheerleader."

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Trump said the United States has become soft and weak like the National Football League, adding that he yearns for the days of "violent, head-on" tackles.

"It was incredible to watch, right?" he said, saying that such tackles now result in a flag being thrown. "Football's become soft. Football has become soft. Now, I'll be criticized for that. They'll say, 'Oh, isn't that terrible?' But football's become soft like our country has become soft. It's true. It's true."

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A few minutes later, Trump wrapped up his speech with a obscenity-filled rallying cry that listed off all the things he would do as president.