Donald Trump is still calling Sen. Ted Cruz (R-Tex.) “a good guy,” but for how long? Starting last week, Trump advanced some unoriginal conjecture about Cruz’s eligibility to run for president. By Sunday, according to the Washington Post, Trump has mixed these comments into his campaign speeches. In the same way that he stoked the anti-Obama “birther” movement, Trump is thinking out loud in a way that encourages his supporters to question Cruz’s eligibility to be president, if not the Texan’s loyalty to his country.

“So, Cruz is a problem," Trump told a crowd in Reno on Sunday, according the Post’s Jenna Johnson who reported on a rally in Reno, NV. "And here's the problem: It's called uncertainty. It's called you just don't know."

Trump has survived modest surges from GOP contenders Jeb Bush and Ben Carson, both of whom have since diminished in the race. Now Cruz is surging, rising to around 18 percent in national polls, according to HuffPo, against Trump’s dominating 36 percent.

Trump’s attacks on Cruz may seem inevitable, though counter-intuitive given their mutual respect in the race so far, culminating, perhaps with their joint rally opposing the White House’s nuclear deal with Iran last fall (the same rally where immigration activists were assaulted and spit on.)

Cruz’s foreign birth, (along with his Hispanic heritage?), allows Trump to attack him on one of their signature issues, immigration enforcement. Cruz has argued that his deportation policy is tougher that Trump’s because it would not allow immigrants currently in the country to ever become U.S. citizens.

In what is becoming a signature Trump style, he has not specified why Cruz is “weak” on immigration. But Trumpeters, as many of the candidate's supporters call themselves, may turn to the Senator's past support of H1b visas, or his opposition to Trump’s proposal to create a special deportation force.

“Is he a natural-born citizen?" Trump asked his supporters in Reno, to cries of “no,” Johnson reported. "I don't know [....] Honestly, we don't know. Who the hell knows."

With the phrase “natural-born citizen,” Trump paraphrases the constitutional requirements of those who are elected to America’s highest office (along with 14 years of residency and a minimum age of 35). Most scholars take the phrase to mean that the person is born a citizen, i.e. did not naturalize through immigration, according to PolitiFact.

If you think you’ve heard Trump rouse rabble by saying “I don’t know” over and over again, you’re right.

“I don't know," Trump told Anderson Cooper in 2011 when asked if he believed Barack Obama was born in the U.S. "I really don't know. I mean, I don't know why he wouldn't release his records. But you know, honestly, I don't want to get in it.”

Trump, of course, really did want to get into it. The same is true of his indirect attacks on Cruz, who is climbing in the polls. As Trump told ABC News in 2013 (speaking about the Obama birth certificate) “I think it made me very popular... I do think I know what I'm doing."