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Donald J. Trump predicted that the lawsuits against Senator Ted Cruz, doubting his constitutional eligibility to be president, would start trickling in as questions continued to percolate about the fact that he was born in Canada. As the Republican candidates gathered to debate in South Carolina on Thursday, one had already been filed.

An 85-year-old trial lawyer, Newton Schwartz Sr., filed the complaint in Federal Court in the Southern District of Texas, in Houston, arguing that the definition of a “natural born citizen” has never been sufficiently settled by the United States Supreme Court. The matter, he said, must be urgently addressed.

“The entire nation cannot afford such constitutional confusion and uncertainties overhanging the electorate process,” Mr. Schwartz, who lives and practices law in Mr. Cruz’s home state of Texas, wrote in the 73-page lawsuit.

Mr. Trump has brought up the issue of Mr. Cruz’s eligibility as the Texas senator’s poll numbers have climbed, arguing that the Republican Party cannot risk the possibility that a Cruz nomination would be tied up in court. Mr. Cruz, along with many legal experts, contend that there is no constitutional basis for such questions. While Mr. Trump has promised not to sue, he has warned on several occasions that Democrats will look to disqualify Mr. Cruz.

Mr. Schwartz appears to be the first to have done so.

“The country would be in chaos,” Mr. Schwartz said in an interview on Friday, in which he discussed the prospect of someone who was not eligible to run being elected president.

Mr. Schwartz said he has no ties to Mr. Trump and that he is not a fan of the billionaire developer. He is, however, a Democrat who voted for President Obama and said he plans to support Senator Bernie Sanders.

Although Mr. Schwartz said that a judge has already been assigned to the case and that he is prepared to take it to the Supreme Court, legal scholars remain skeptical that it could get that far.

Peter Spiro, a law professor at Temple University who reviewed the filing, noted that the arguments were meandering and the language was full of typographical errors. But of greater concern, he said, is the probability that Mr. Schwartz does not have legal standing to bring such a case against Mr. Cruz.

Many agree that the definition of a “natural born citizen” — a constitutional requirement for seeking the presidency — remains muddled. The issue has not been decided by the courts because most lawyers think it would require a presidential candidate, or an opponent of one who can claim “injury,” for courts to consider the merits of the case. Mr. Schwartz argues in his complaint that because he is a voter, he has standing to bring the lawsuit.

According to Mr. Spiro, being a voter is too broad a classification to have standing and that the most realistic situation that would bring legal clarity would be for Mr. Cruz to file a lawsuit in the event that a state elections commissioner decided to take him off the ballot because of his Canadian roots. The suggestion that Mr. Trump has made, urging Mr. Cruz to seek a “declaratory judgment,” is also unrealistic, Mr. Spiro said, because courts do not offer such guidance unless there is an active case.

“There’s zero chance of a court getting to the merits on this claim,” Mr. Spiro said. “If Trump wants to put his money where his mouth is, he should be the one filing for a declaratory judgment against state election commissioners who put Cruz’s name on the ballot.”