He sees his struggle in the tradition of the grass-roots fight for divestment from apartheid South Africa, as well as the American civil rights movement. Some critics see him as anti-Semitic, or else as allowing his political bias to undermine the education of his students.

The clashing visions turn on a reference letter, one of the most valuable currencies of the teacher-student relationship. At the University of Michigan, the letter of recommendation is now also a tool in the protest against Israel, as John Cheney-Lippold, a professor of cultural studies, this month rescinded his offer to write on behalf of his student’s semester abroad at Tel Aviv University.

His decision, first reported by the Michigan Daily campus newspaper, newly tests the line between opposition to Israel and hostility to Jews, while marking the latest chapter in the bitter debate over the movement known as BDS — Boycott, Divestment and Sanctions. The movement seeks the end of Israeli occupation of “all Arab lands,” the full equality of Arab-Palestinian citizens of Israel and the right of return for Palestinian refugees as stipulated in United Nations General Assembly Resolution 194.

A scholar of digital media, identity and privacy, Cheney-Lippold has the academic chops to examine how cultural debates ignite anger that spreads online, threatening those who take a controversial stand. In this case, the stand is his own. And the debate is hardly an academic abstraction.

It originated in his own email inbox, where a query from a student arrived in August. The student’s request was a standard one, made of professors around the world. After a back-and-forth, in which he asked for a clearer deadline from the student, identified by the Michigan Daily as a junior at Michigan’s College of Literature, Sciences and the Arts, Cheney-Lippold agreed to write on her behalf for a study-abroad program.

But when he received the form letter, Cheney-Lippold realized that he had missed a key detail. His student’s desired destination was Israel, whose academic institutions he has pledged to boycott as a way of protesting the state’s treatment of Palestinians. Cheney-Lippold is a member of the American Studies Association, whose members in 2013 voted by a ratio of more than 2 to 1 to endorse BDS.

He wrote to the student on Sept. 5 to say he wouldn’t write the letter.

“I am very sorry, but I only scanned your first email a couple weeks ago and missed out on a key detail,” wrote Cheney-Lippold, who had been conducting research at London’s British Library, when the initial request came in. “As you may know, many university departments have pledged an academic boycott against Israel in support of Palestinians living in Palestine. This boycott includes writing letters of recommendation for students planning to study there.”

He told her he would be happy to recommend her for other programs.

Eleven days later, controversy erupted when Club Z, a Zionist youth movement based in California, posted an image of the Sept. 5 email on its Facebook page, labeling the professor’s message “unbelievable” and saying it should be investigated for anti-Semitism. The pro-Israel youth group clarified in a Facebook comment that it had obtained Cheney-Lippold’s email “from another university professor, to whom the email was sent.”

“Religious discrimination,” one user commented. “Fire immediately.” Others defended the professor: “No one is entitled to a recommendation letter. It is simply a good gesture on behalf of the faculty.”

Among those who weighed in on social media was Anti-Defamation League CEO Jonathan Greenblatt, who declared the student a “victim of political bias.” The ADL calls BDS a “campaign to delegitimize Israel” and says it is “rampant with misinformation and distortion.”

Cheney-Lippold said the situation of Palestinians in Israel has become acute with the passage this summer of the nation-state law declaring the country a national homeland for the Jewish people. Academics, he said, should be particularly concerned about the treatment of minority identity groups at state universities.

Still, he would come to acknowledge an error in his email. “While many of the people in my faculty support me, the department itself doesn’t have a position on BDS, nor does any other department at the University of Michigan,” he said in an interview with The Washington Post.

The university’s governing body, the Board of Regents, declined last year to form a committee to investigate divesting the university’s endowment from companies doing business with Israel, after the student government passed a resolution supporting such a move.

Only twice in its history has the University of Michigan divested — from apartheid South Africa in 1978 and from the tobacco industry in 2000. Last year, Michigan Gov. Rick Snyder, a Republican, signed into law a measure prohibiting boycotts of individuals or public entities of a foreign state, though the legislation did not refer specifically to Israel. In this case, a university spokeswoman said, “The university has consistently opposed any boycott of Israeli institutions of higher education.”

The student, who didn’t return a Facebook message seeking comment, “has asked that we respect this as a private matter,” said the spokeswoman, Kim Broekhuizen.

Cheney-Lippold said he has received a flood of phone and email messages, including death threats.

He had been careful in wording his email, wanting to impress upon the student that his decision was not personal. He rewrote the message twice to perfect its tone. But the choice was otherwise a simple one. “It was about consistency,” he said. “If I believe in this, I have to exercise my will as a professor.”

“If a union asks me not to buy a grape from a certain producer, or not to cross a picket line, I would support that,” he said. “It’s the same thing here. Following requests from Palestinian and Jewish activists, I find the boycott against Israeli state institutions to be a very useful way to put pressure where I can as an academic.”

His support for the boycott — an international protest that has been criticized as inhibiting academic freedom and free expression — did not interfere improperly with his student’s plans, he noted. Nor has his involvement been inconsistent with his teaching duties, he said, but rather is protected by his own academic freedom. “I can’t prevent a student from going to Israel,” Cheney-Lippold reasoned. “But everybody has the right to withhold something, and I chose to exercise that right based on what the movement needs from me as a solidarity activist.”

His university sees matters differently.

“Injecting personal politics into a decision regarding support for our students is counter to our values and expectations as an institution,” the university spokeswoman said.

But professors decline to write letters of recommendation for a range of reasons, Cheney-Lippold said, including competing obligations and knowledge of the student.

“The reason why my case became viral is because I was honest with her,” he said. “I think she deserved honesty. But I didn’t realize declining to write a letter would have such a political effect.”

The reason it touched a nerve, he suggested, is not just because of the vexed debate over the Israel-Palestine conflict but also because of a misunderstanding of free speech and a professor’s role. He argued that rising tuition means a college education is increasingly understood as an investment, and a letter of recommendation as something owed to a student as a consumer. “Michigan’s brand is being stained right now,” he said.

Cheney-Lippold said he hasn’t met with the “upper echelons” at the university, but that his department chair has been supportive.

He has also been able to turn the dispute into a valuable teaching opportunity, he said. Cheney-Lippold, who is the author of “We Are Data: Algorithms and the Making of our Digital Selves,” is teaching two courses this semester, one on cultural studies “from origins to the Internet,” and the other on “the politics of code.”

In both class meetings on Tuesday, he said, he opened the floor for questions and discussion. Some of his students were critical of him, he said. Others affirmed their support.

“A Jewish student, a student who identified himself as Jewish, said, ‘All of my friends were calling this professor an anti-Semite, and I told them he’s the furthest thing from an anti-Semite,’ ” Cheney-Lippold recounted.