Because of the vacancy, he says, "the Second Amendment, life, marriage, religious liberty, every one of those hangs in the balance." He warns that "we cannot trust Donald Trump" with this responsibility.
We don't know what sort of person Trump would like for the court. But we do know what sort of person Cruz likes. And based on that knowledge, we can't trust Cruz -- by Cruz's own standards.
Consider two justices who have elicited strong opinions from him. The first one is someone whose appointment he regards as "a mistake," who in one landmark case "changed the law in order to force that failed law on millions of Americans for a political outcome." The second is a "principled conservative" known for "faithfully applying the Constitution and legal precedent."
You may conclude that the Texas senator has a sharp eye for the right kind of justice and the wrong kind. Not quite. Both his praise and his condemnation were in reference to the same person: John Roberts.
When Roberts was nominated in 2005, Cruz gushed over him. Today, he regards the chief justice, who had the gall to uphold Obamacare, as part of "an out-of-control court." Either Cruz was wrong before, or he's wrong now.
It could be that Cruz is not very good at detecting which Supreme Court prospects are principled conservatives. Or it could mean that he is good at picking them out -- but clueless on what their approach will produce.
Maybe Roberts is not a conservative, or maybe a conservative path doesn't always lead where Cruz wants to go. However, it's hard to make the case that he's not a conservative: During the court's last term, Roberts voted the same way as Scalia 90 percent of the time -- almost exactly as often as Clarence Thomas did.
When Roberts voted to uphold Obamacare, it's possible he was being faithful to his principles, at the price of offending his usual allies. That would be in keeping with what Scalia once said: "Very often, if you're a good judge, you don't really like the result you're reaching."
Republicans often act as though there is a clear, simple, conservative approach to judging -- which is to apply the words of the Constitution according to their original meaning. That's the mode of judging (known as originalism or textualism) that Scalia championed. But his approach never won over even most of his Republican-appointed colleagues.
Roberts, Samuel Alito and Anthony Kennedy, notes University of Chicago law professor Eric Posner in Slate, "are not originalists." They don't go strictly by what the text was understood to mean when it was written but "decide cases the way justices always have: by using whatever materials at hand -- historical sources, yes, but also (and mainly) judicial precedents, common sense, general principles, political values and so on."
Nor does originalism necessarily tilt rightward. Liberals like Yale's Akhil Reed Amar and Jack Balkin embrace it. So did Supreme Court Justice Hugo Black, a hero of the American Civil Liberties Union.
Scalia himself didn't even practice it all the time. He voted to strike down a law against burning the American flag, which he saw as a violation of the First Amendment right to free speech. But flag-burning is not speech -- only a form of communication -- and the opinion joined by Scalia made no attempt to prove the framers thought it was.
The more important fact, though, is that even staunch conservatives can reach drastically different answers to fundamental questions. Scalia wrote the decisions interpreting the Second Amendment to protect an individual right to own guns.
But Appeals Court J. Harvie Wilkinson III -- an undisputed conservative appointed by Ronald Reagan -- said it was wrong to "read an ambiguous constitutional provision as creating a substantive right that the court had never acknowledged in the more than 200 years since the amendment's enactment."
Cruz promises that any justice he chooses "will faithfully follow the law." But all justices think they do that. Roberts is a villain in Cruz's eyes only because he ruled in a way different from what Cruz wanted.
It would be a terrible justice, though, who would slavishly implement the desires of the president who made the appointment. If Cruz wants a justice who will always rule the way he prefers, he's chasing the wrong job.