Boston — Alan Dershowitz - the Harvard law professor and noted ''devil's advocate'' who champions the unpopular and unfriendly in court - was sitting in a restaurant with his children recently when he saw a flier headlined, ''Boston Against Blacklisting.''
''I immediately wanted to attend this benefit. I'm against blacklisting,'' Professor Dershowitz commented later. ''Then I saw who it was for. It was for Vanessa Redgrave.''
So Dershowitz stood outside the theater, handing out fliers of his own, alleging that Miss Redgrave supports blacklists in England.
The name and politics of Vanessa Redgrave, currently involved in a court case here, can do that to people.
Redgrave has long been a supporter of the Palestine Liberation Organization. In 1978 she aroused controversy by narrating two films sponsored by the PLO. Her anti-Zionist statements since then have aroused the ire of Jewish leaders across the United States and abroad.
The tall, sometimes austere actress put a controversial motion before the British actors' union that would have enjoined members from traveling to Israel. This move led to the charge that she supports blacklists. She defends it by saying she thinks Israel should be boycotted for Zionism - which she equates with racism - just as South Africa has been boycotted by artists and athletes.
In 1980, CBS was flooded with protests when Redgrave was cast as a half-Jewish orchestra leader in Arthur Miller's television drama ''Playing for Time.''
The current headlines, of course, have to do with Redgrave's suit against the Boston Symphony Orchestra for firing her in 1982 as narrator of Stravinsky's ''Oedipus Rex'' when protests erupted in the Jewish community here after the engagement was announced.
But the issues in the case reach far beyond Redgrave and the BSO. Observers here and elsewhere see the case as another flare in a controversy that extends throughout the arts and across the centuries.
''The mixing of politics and music goes way back to the Greeks,'' comments one Boston musicologist who prefers to remain anonymous. ''Certain types of music have been banned throughout history. There is, in fact, a great body of political music.''
It could be argued that Beethoven's intention of dedicating his third symphony to Napoleon was a strongly political act in a Vienna that looked with extreme nervousness on events in France at the time. In addition, there were political implications in Verdi's operas, such as ''Nabucco,'' which symbolized resistance to Austrian domination.
The pages of musical history are at least footnoted, and sometimes filled, with political battles.
The controversy between Vanessa Redgrave and the BSO may seem small by comparison, but it drives into the heart of the confrontation between art and politics.
Redgrave's charge that she is a victim of ''blacklisting'' gains some credence from Theodore Mann of the celebrated Circle in the Square Theatre in New York City, who states categorically that the BSO firing led his company to reconsider hiring her for its production of ''Heartbreak House.''
The orchestra has argued that it acted to protect the musicians and audience from threatened violence, and that the potential loss of economic support was only secondary.
The case is being decided before a jury - as it will also probably be in an appeal - on the technical grounds of whether a breach of contract took place and whether, under a five-year-old Massachusetts civil rights statute, Redgrave's rights were violated.
But the issue of arts and politics is also being tried across this city - in the pages of Boston newspapers, in living rooms, and on a theater stage, during a benefit performance by Miss Redgrave and others. It has set people who normally agree against each other. It revolves around a single issue:
Should political beliefs exclude an artist from being able to perform anywhere in a free society?
''The issue is that Vanessa Redgrave is a good actress,'' argues composer Earl Kim, professor of music at Harvard University and a respected composer. ''She was hired as a good actress, and she agreed to do it. Then she was fired for her politics. For me that is the only issue.
''If you ask me about her politics, I could say, 'They stink,' '' he adds. Nonetheless, he considers the firing of Redgrave ''a very, very dangerous thing: It reminds us a little of the McCarthy era, when people lost their jobs because of their political beliefs,'' adds Professor Kim, who was himself blacklisted during the red scare of the '50s.
''I disagree that politics and music must live together,'' BSO conductor Seiji Ozawa testified, arguing in defense of the orchestra's decision to fire Redgrave.
Mr. Ozawa, however, conducted a Symphony Hall concert last year to benefit Musicians Against Nuclear Arms, an organization designed to help passage of a nuclear freeze amendment in the US Congress.
Not that he flew in the face of contemporary practice by doing so. Politics in the concert hall are common enough: from music based on the poetry of a Soviet dissident today, to political speeches given by Paul Robeson at concerts decades ago.
But those who support Redgrave's firing see an essential distinction:
''Robeson was different,'' says one Boston musicologist. ''He was a communist. And certainly he advocated revolution, but not the violent overthrow of a state, as Redgrave does. ...'' After thinking for a moment, he observes, ''There were similar occurrences for political reasons in the '30s, when artists wouldn't participate (in performances). ... The disagreements between the Stalinists and the Trotskyites were unbelievable.''
Indeed, they were. And, although the conflict between the followers of Trotsky - who advocated a world-scale communist revolution in violent disagreement with the nationalistic programs of Stalin - involved a tiny minority, the debate over art and politics boiled then.
George Orwell crystallized this debate in a defense of his book ''Homage to Catalonia.'' It contains a chapter full of newspaper quotations defending the Trotskyites who were accused of plotting against Franco. One critic complained to Orwell: ''You've turned what might have been a good book into journalism.'' Orwell wrote, ''What he said was true, but I could not have done otherwise. I happened to know, what very few people in England had been allowed to know, that innocent men were being falsely accused. If I had not been angry about that I should have never written the book.''
Now another Briton - who resembles Orwell intellectually, has run for Parliament as a member of the Revolutionary Workers Party, and is herself a Trotskyite - feels convinced that she has seen things the general public does not know.
Her views have set a pot of controversy to boil in Boston. Redgrave and her supporters are not above fanning the fires under that pot.
In the heat of the trial, they staged a provocative evening of scenes and songs, drama and music that evoked the specters of McCarthyism, the Salem witch trials, persecution of Jews in Poland, and the Scopes ''monkey'' trial. The production, which featured the towering talent of Vanessa Redgrave center stage, brought more heat than light to an issue that many observers here feel has been plunged in darkness.
Perhaps this was because the issue reaches so far beyond the name and politics of Vanessa Redgrave and the emotions they inspire.