And, Cruz would suggest, Trump himself should be prepared to make his case in court.
A federal lawsuit challenging Cruz's eligibility to run for president was filed Thursday, based on the theory espoused by Trump that he may not be a natural-born citizen.
But Cruz is not alone -- his fellow Cuban-American presidential candidate and freshman senator is facing similar litigation.
Cruz was conferred American citizenship at birth because his mother is an American citizen, and legal experts have largely agreed that would qualify him for natural-born citizenship. The Texas Republican was born in Calgary, Alberta, Canada, and also had Canadian citizenship until he renounced it in 2014.
Schwartz cites legal theorists, including Harvard law professor Laurence Tribe, who say although Cruz is generally accepted to be a natural-born citizen, no court has definitively ruled on the question.
The move follows weeks of attacks by the GOP front-runner on Cruz's status, which came to a head during Thursday's GOP debate. Trump told Cruz he was a liability as a candidate if he runs for president, because his eligibility will be tied up in lawsuits for years.
But though Trump isn't attacking him, Rubio is facing similar challenges, which Cruz obliquely referenced during the debate on Thursday, as well.
Both candidates have had their spot on the ballot in Illinois challenged after the recent primary filing deadline. The same man, William K. Graham, accused both of not being eligible natural-born citizens, according to a spokesman for the Illinois Board of Elections. Cruz was challenged on his place of birth, and Rubio on both of his parents not being U.S. citizens at the time he was born.
The challenges will likely be decided by February, the spokesman said, but there's no historical precedent that makes clear how it will be decided. Unlike President Barack Obama's challenge, which was about his place of birth, "this is a question of interpretation of the law, and as far as we know, it hasn't been decided yet."
A similar lawsuit
was filed in Broward County, Florida, in December, challenging Cruz and Rubio on the same grounds.
Rubio's legal team responded
to that challenge on Monday, as first reported
by the Tampa Bay Times, saying it is "undisputed" that the Florida Republican was born in the U.S., making him a natural-born citizen.
Trump also spread many of the birther theories that hung over Obama when he ran for office, fueled by conspiracy theorists who did not believe that Obama was born in the U.S., as he was. Obama's mother was also a U.S. citizen at his birth, and no credible legal challenge to his eligibility ever progressed in the courts.
Legal experts believe the same will be true of the Cruz challenge.
Asked if Schwartz had standing -- the legal principle that requires an individual to have suffered some harm to bring a lawsuit -- American University law professor and CNN contributor Stephen Vladeck said "not even a little."
"Standing's only going to work if some state denies Cruz access to the ballot," Vladeck said, at which point Cruz himself could sue to have it resolved.
Vladeck also predicted the case would never reach the Supreme Court.
"It's the epitome of what they call a generalized grievance," he told CNN's "Legal View with Ashleigh Banfield."
Rubio's team also challenged the Florida suit on the grounds of standing. Cruz has not yet responded to the suit.
Rubio's team also noted the man bringing that lawsuit, Michael Voeltz, brought a lawsuit challenging Obama's natural-born citizenship on the same grounds, that both of Obama's parents weren't U.S. citizens. That case was dismissed.
Rubio has faced such
challenges since 2011, when birthers began questioning his citizenship.
For his part, Schwartz told "Legal View" that while he is a Bernie Sanders supporter, he would be happy with any outcome in the case.
He said the biggest issue is that the definition of natural-born citizen is too unsettled.
"If the Supreme Court goes against me and (Cruz is) eligible ... that satisfies my purpose," Schwartz said. "It's the most ambiguous provision and it hasn't been tested in 220 years. Now the time has come to have a case for testing it."
Cruz also mentioned the fringe legal theory that requires both parents to be U.S. citizens onstage Thursday night, using it call into question even Trump's citizenship.
"I would note that the birther theories that Donald has been relying on -- some of the more extreme ones insist that you must not only be born on U.S. soil, but have two parents born on U.S. soil," Cruz said. "Under that theory, not only would I be disqualified, Marco Rubio would be disqualified, Bobby Jindal would be disqualified and, interestingly enough, Donald J. Trump would be disqualified."
Trump's mother was born in Scotland.
"But I was born here," Trump shot back.
Cruz has maintained he is a natural-born citizen.
"Back in September, my friend Donald said he had his lawyers look at this in every which way," Cruz said in Thursday night's debate. "There was nothing to this birther issue."
"Since September, the Constitution hasn't changed," Cruz added. "But the poll numbers have."
The birther movement has long roots, including challenges to former Republican presidential candidate Sen. John McCain, who was born on a U.S. military base in Panama, and Republican George Romney, Mitt Romney's father, who was born to U.S. citizens in Mexico.