Donald Trump Revives 'Birther' Debate, and Ted Cruz Tweets Fonzie Jumping the Shark

Sparring between the Republican Party's top two presidential candidates is heating up amid Donald Trump's revival of "birther" questions surrounding Ted Cruz's eligibility to hold the US's top office.

Trump, the GOP's billionaire businessman frontrunner, who once falsely fueled claims that President Barack Obama was born in Kenya, has now set his sights on Cruz, the Texas senator who is currently leading the field in Iowa. While the pair may have previously paraded their mutual admiration on the stump, a public tiff this week over Cruz's birthplace (Canada) may signal the end of the bromance.

"Republicans are going to have to ask themselves the question: 'Do we want a candidate who could be tied up in court for two years?' That'd be a big problem," Trump said in an interview with the Washington Post published Tuesday. "It'd be a very precarious one for Republicans because he'd be running and the courts may take a long time to make a decision. You don't want to be running and have that kind of thing over your head."

"I'd hate to see something like that get in his way. But a lot of people are talking about it and I know that even some states are looking at it very strongly, the fact that he was born in Canada and he has had a double passport," he added.

Cruz, who has frequently spouted quotes from the Princess Bride while campaigning or referenced Dr. Seuss on the Senate floor, retorted to Trump's comments with another pop culture reference — this time tweeting a scene from the sitcom Happy Days.

My response to — Ted Cruz (@tedcruz)January 5, 2016

The two-and-a-half minute clip is drawn from the episode "Fonzie Jumps the Shark" (1977). The particular sketch gave rise to the term "jumping the shark" — popular jargon for a struggling TV show that exploits an outrageous gimmick to resuscitate popularity or audience interest.

Questions of Cruz's ability to assume the Oval Office in 2016 began swirling months before he first announced his candidacy in March 2015. Under the US Constitution, a person needs only three qualifications to be president: She or he must have resided in the US for 14 years, be at least 35 years old, and be a "natural born citizen."

Related: Wait… Can Canadian-Born Ted Cruz Actually Become the Next US President?

Cruz was born on December 22, 1970, in Calgary to an American mother and Cuban-born father, who became an American citizen in 2005 and is now a considerable conservative figure himself, as well as a suburban pastor in Dallas. The younger Cruz later renounced his Canadian citizenship — granted automatically for babies delivered on Canadian soil — in May 2014, after a Dallas paper reported that unnamed detractors were already pegging him as "Canadian Ted," and questioning his ability to run for the presidency.

Cruz, who later studied law at Harvard and became a clerk for the US chief justice, maintains his US "natural born" status was automatically conferred to him by his American mother, who has met the requisite US residency requirement of 10 years, with five of those being after the age of 14.

"I was born in Calgary. My mother was an American citizen by birth," Cruz said at the annual Conservative Party Action Conference this February. "Under federal law, that made me an American citizen by birth. The Constitution requires that you be a natural born citizen."

Sarah Duggin, a law professor at Catholic University, told VICE News in March that while it may not be as clear-cut as all that, many constitutional law experts agree Cruz is eligible.

"The argument is that we don't know what [the founding fathers] meant by that phrase 'natural born' citizenship," Duggin said. "Some argue they meant very specifically that you have to be born physically in the US to be eligible to serve as president, but essentially it could also include someone who was 'naturalized at birth' from being born to an American parent in another place."

"[The latter is] the better argument, and the one most scholars and political academics agree with," she added. Yet, without a specific Supreme Court ruling on the matter or an amendment to clarify the Constitution, "there is no open and shut information," she said.

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While even a hint of ambiguity remains, it's likely that challengers will file suits over Cruz's "natural born" status, just as they did with another Republican senator running for president, John McCain, Duggin said. McCain (who once called Cruz a "wacko bird" and "crazy") faced similar scrutiny over his birth, and was forced to defend his eligibility for the presidency in 2008 after receiving the Republican nomination in March of that year. He was born in Panama from American parents.

Obama has also received his fair share of challenges and federal lawsuits filed persistently throughout his two terms in office, based on conspiracy theories he was actually born in Kenya, not Hawaii, and is therefore ineligible to hold the office of president.

Ultimately, it is "very difficult to litigate" cases about the constitutional eligibility of candidates, Duggin said, adding that any similar cases against Cruz will probably be thrown out.

"It's likely courts will say this is not something we want to get involved in, and so far the lower courts have refused to get into the merits," she said.

Follow Liz Fields on Twitter: @lianzifields