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OTTUMWA, Iowa — Donald J. Trump sharply escalated his rhetoric about Senator Ted Cruz’s eligibility to be president on Saturday, suggesting that because he was born in Canada there were unanswered questions about whether he met the constitutional requirement to be a “natural-born citizen.’’

“You can’t have a person who’s running for office, even though Ted is very glib and he goes out and says ‘Well, I’m a natural-born citizen,’ but the point is you’re not,” Mr. Trump said while campaigning in Clear Lake, Iowa.

Mr. Cruz was born in Calgary, Canada, to an American mother, which automatically conferred American citizenship. Most legal experts agree that satisfies the requirement to be a “natural-born citizen,’’ a term that was not defined by the founders.

Mr. Trump, who began raising questions about Mr. Cruz’s ability to be president earlier in the week, said on Saturday that Mr. Cruz would have to go to court to get a “declaratory judgment” about his eligibility “or you have a candidate who just cannot run.’’ (Mr. Cruz could need a judgment if someone filed a lawsuit to challenge his candidacy and a court agreed to take up the question.)

With polls showing the race in Iowa tightening, and Mr. Cruz leading Mr. Trump by 4 percentage points in a Fox News poll released on Friday, Mr. Trump has returned to an issue that first gained him notoriety years ago when he challenged President Obama’s citizenship.

On Saturday night, before the final stop on a six-day bus tour of Iowa, Mr. Cruz said: “Under longstanding federal law, the child of a U.S. citizen born abroad is a natural-born citizen.”

“You’re seeing candidates trying to throw whatever rocks they can,” he said. “That’s fine, that’s their prerogative. I like Donald Trump, I respect Donald Trump. He’s welcome to toss whatever attacks he wants.”

Mr. Trump’s criticism of his rival did not stop there. At an earlier rally in Ottumwa, Iowa, he accused Mr. Cruz of flip-flopping on his support of ethanol, a fuel that can be made from corn and a major industry in Iowa.

“As you know my primary opponent was totally opposed to ethanol and the ethanol industry,’’ Mr. Trump said. “And all of a sudden he was getting clobbered. All of a sudden he said, uh, I’m for ethanol. You can’t do that. You can’t do that with three weeks. You’re not allowed to do that.’’

All year, Mr. Cruz has defied political convention by calling for the end of federal support for ethanol. He has been asked about it repeatedly by voters on his bus tour, all while being trailed by a pro-industry group in a recreational vehicle pressuring him to change his views.

Mr. Cruz said this week that he favors a five-year period to phase out the federal mandate requiring fuel companies to incorporate a certain amount of biofuels, like ethanol, into gasoline. The pro-industry group following him, America’s Renewable Future, claimed a victory. Its director, Eric Branstad, said in a statement: “He started the caucus process calling for immediate repeal. While not perfect, this is a big step forward by Sen. Cruz.”

But the Cruz campaign said the group was misrepresenting the senator’s position, pointing to legislation Mr. Cruz sponsored that called for a five-year phase phase-out of the ethanol standard.

Although ethanol is an important economic issue for farmers, its power in politics seems to be waning in Iowa. Mr. Cruz’s well-publicized opposition has not kept him from leading in polls.

While he did not criticize Mr. Trump explicitly, Mr. Cruz seemed to allude to him when he spoke of campaign strategy on Saturday night.

“The only way to win Iowa is one voter at a time, showing the voters, the men and women of Iowa, the respect to look them in the eyes, to answer their questions,” Mr. Cruz said. “I don’t believe any candidate will win the state of Iowa, and I don’t believe any candidate will win the state of New Hampshire from a TV studio in Washington, D.C., or in Manhattan.”

Fact Check: Donald Trump Raises Questions About Ted Cruz's Citizenship Mr. Trump raised questions about whether Ted Cruz was eligible to be president because he was born in Canada. Not a problem, legal experts say. See all Election 2016 fact checks » Mr. Trump, in interviews this week, suggested that Mr. Cruz’s birthplace could invite legal challenges. “It’s a problem for him, and it’s a problem obviously for the Republicans,” Mr. Trump said on MSNBC’s “Morning Joe.” Critics of Mr. Cruz have questioned his eligibility to be president for some time. Senator John McCain, who was born in the Panama Canal Zone, faced similar questions when he ran for president in 2008. Mr. Cruz was born in 1970 in Calgary, Alberta, to an American-born mother and a Cuban-born father. His parents were in Canada working in the oil business. Mr. Cruz was an American citizen at birth because of his mother’s citizenship. The Constitution, in Article II, requires the president to be a “natural born citizen.” But the Constitution does not define that term, which is why Mr. Cruz, and other presidential candidates before him, have faced questions about their eligibility. The general view among legal scholars is that “anyone who acquires citizenship at birth is natural born for purposes of Article II,” as Sarah Helene Duggin, a law professor at Catholic University, wrote in 2013. Paul D. Clement, who served as solicitor general in the administration of President George W. Bush, and Neal K. Katyal, who served as acting solicitor general in the Obama administration, took up the subject in the Harvard Law Review last year. To understand the term “natural born citizen” as written by the framers of the Constitution, they pointed to British common law and legislative action by the First Congress. “Both confirm that the original meaning of the phrase ‘natural born Citizen’ includes persons born abroad who are citizens from birth based on the citizenship of a parent,” they wrote. In Mr. Cruz’s case, they concluded that “despite the happenstance of a birth across the border, there is no question that Senator Cruz has been a citizen from birth and is thus a ‘natural born Citizen’ within the meaning of the Constitution.” — Thomas Kaplan Close