For many of us, the highlight of the latest Republican debate was Ted Cruz demolishing Donald Trump’s attack on his status as a natural-born citizen eligible for the presidency. For added insult to injury, Cruz pointed out the following about the constitutional “expert” Trump has repeatedly cited on the matter:

Let me tell you who Larry Tribe is. He’s a left-wing judicial activist, Harvard Law professor who was Al Gore’s lawyer in Bush versus Gore. He’s a major Hillary Clinton supporter. And there’s a reason why Hillary’s supporters are echoing Donald’s attacks on me, because Hillary wants to face Donald Trump in the general election.

This was devastating not merely for discrediting the legal question, which was never going to be a serious problem, but for demonstrating that Trump is getting his information from liberals and has no idea what an actual conservative would consider a credible authority. Indeed, Tribe prefaces his analysis of the case with an ode to “living Constitution” judges who “believ[e] that the Constitution’s meaning evolves with the perceived needs of the time and longstanding practice”—which anyone who came to conservatism naturally would instantly recognize as code for twisting the Constitution to justify whatever liberals want.

But it turns out there’s another reason why The Donald choose poorly: his expert is not just mistaken, but lying. In 2008, Tribe told PolitiFact that John McCain’s citizenship “suffice[d] to make him a natural born citizen” eligible for the presidency despite being born in Panama, citing the Naturalization Act of 1790, which says the children of American citizens “born beyond the sea or out of the limits of the United States, shall be considered as natural born citizens.”

And no, this circle can’t be squared by invoking the living Constitution. Tribe now claims the question is “completely unsettled,” that “the legal principles that prevailed in the 1780s and ’90s required that someone actually be born on US soil,” but back then he specifically said he based his contrary assessment “on the original meaning of the Constitution” and “the Framers’ intentions.”

He was right the first time, as actual legal experts like Andrew McCarthy, Elizabeth Price Foley, and John Eastman would be happy to explain to Trump. The English common law definition encompasses children born to one nation’s subjects on another’s soil, Father of the Constitution James Madison recognized “allegiance” as a higher criterion for natural-born citizenship that geographic location, and Cruz’s mother met all the requirements of US law at the time of his birth.

And because Tribe suffers from the twin left-wing demons of thinking himself untouchable and being unable to resist overreach, he caps off this shameless peddling of legal theories he doesn’t really believe with the accusation that Cruz is the “constitutional opportunist” here.

So Trump’s new favorite attack rests on premeditated constitutional malpractice. Trump probably doesn’t know that, but that’s another symptom of his standard operating procedure: sheer laziness. He doesn’t care enough to bother with thinking attacks through or even checking whether they’re true; he just grabs whatever sounds damaging. That’s why the visibly-rattled Trump is following up by charging “nobody in Congress likes” Cruz because he’s “nasty”—without realizing he’s reciting an old favorite of the very Republican establishment Trump’s supporters are revolting against—and inflating the non-story over Cruz’s Goldman Sachs loan into the absurd warning that Cruz “will do anything they demand” because “Goldman Sachs owns him.”

Donald Trump’s candidacy has survived for so long because most competitors who’ve attacked him have done so from his left on positions where Trump has the pulse of the Republican base. But now Trump is the one going out of his way to attack from the left-wing playbook, and he’s picked a target the conservative base considers one of them. So far efforts to publicize his past liberal positions have mostly fallen on deaf ears because his supporters don’t trust most of the alternatives anyway, but seeing Trump act like a leftist here and now could be the silver bullet that gets his true colors to finally sink in.