Ted Cruz's office says "in the mid-1990s, the Harvard Law School faculty included numerous self-described proponents of 'critical legal studies'..."

"... a school of thought explicitly derived from Marxism – and they far outnumbered Republicans."

That's in response to a New Yorker article quoting something Cruz said in a speech 3 years ago. (What Cruz said back then, at an Americans for Prosperity conference, was that when he was at Harvard Law School "There were fewer declared Republicans in the faculty when we were there than Communists! There was one Republican. But there were twelve who would say they were Marxists who believed in the Communists overthrowing the United States government.")

The Cruz spokesperson called it "curious that the New Yorker would dredge up a three-year-old speech and call it 'news.'"

Curious... there's a noncommittal word. I don't see anything wrong with digging stuff out of old Cruz speeches. He's a new character on the national stage, so it's not like old territory is being reworked. It was an inflammatory statement, and he needs to stand by it (and back it up), defend it as hyperbole, or concede he was wrong.
The New Yorker writer, Jane Mayer, was following up after Barbara Boxer had compared Cruz to Joseph McCarthy. That was pretty inflammatory too (as I said at the time). What Boxer said made it a valid line of inquiry for Mayer and not odd at all. What you say to your base will be heard by the outsiders too, and any politician needs to be prepared for that. Republicans hoping for a new star better not forget how badly Mitt Romney faltered when he had to deal with the 47% remark he'd used on the insider group. This Cruz quote is the same kind of thing. Don't minimize it.

Mayer talked to Charles Fried, the Harvard lawprof who was probably the one Republican referred to by Cruz. Fried says:

"I have not taken a poll, but I would be surprised if there were any members of the faculty who ‘believed in the Communists overthrowing the U.S. government".... Fried acknowledged that "there were a certain number (twelve seems to me too high) who were quite radical, but I doubt if any had allegiance or sympathy with anything called ‘the Communists,’ who at that time (unlike the thirties and forties) were in quite bad odor among radical intellectuals.” He pointed out that by the nineteen-nineties, Communist states were widely regarded as tyrannical. From Fried’s perspective, the radicals on the faculty were "a pain in the neck." But he says that Cruz’s assertion that they were Communists “misunderstands what they were about."

Clearly, it was rhetoric to call the Critical Legal Studies professors "Marxists" who believed in "Communist" revolution, and Cruz chose to do that at a particular place and time. Cruz is accountable for that. It's a shibboleth of the right to rely on the words "Marxist" and "Communist." It wasn't the way the lefty lawprofs of the time talked about themselves. I have a vivid memory of saying to a CLS lawprof — a very good friend, during a casual conversation — "I'd like to know about the connection between CLS and Marxism." She snapped: "There's none." I got the message: You sound right wing. It was understood that to sound right wing was to become toxic.

Here's a useful passage from the classic 1983 CLS book by Harvard lawprof Duncan Kennedy, "Legal Education and the Reproduction of Hierarchy: A Polemic Against the System":

Left-liberal rights analysis submerges the student in legal rhetoric, but, because of its inherent vacuousness, can provide no more than an emotional stance against the legal order. The instrumental Marxist approach is highly critical of law, but also dismissive. It is no help in coming to grips with the particularity of rules and rhetoric, because it treats them, a priori, as mere window dressing. In each case, left theory fails left students because it offers no base for the mastery of ambivalence. What is needed is to think about law in a way that will allow one to enter into it, to criticize without utterly rejecting it, and to manipulate it without self-abandonment to their system of thinking and doing.