No matter how low your expectations for contemporary liberalism start, or how fast and far you lower them, there is no keeping up with the degradation of that dogma, as in the scandal of ''Miss Saigon.'' The infrequency and inadequacy of liberal thinking is now threatening to take a new toll by darkening the Broadway theater in which ''Miss Saigon'' was to open next March.

That musical has thrilled audiences in London's West End. But in an act of real moral heroism, the producer has announced cancellation of the Broadway run rather than tolerate the liberal racism of an American union, Actors' Equity, which wants to dictate the racial composition of the cast -- in one crucial particular.

''Miss Saigon'' is set in Saigon in the waning hours of the Vietnam War. The lead part portrays a pimp of Eurasian parents -- French and Vietnamese. Last week, the exquisitely misnamed Actors' Equity voted to deny Jonathan Pryce, who played the lead in London, the right to play it in New York.

The problem with Pryce is that he is not Asian. Of course neither is the character he plays. But the tender conscience of Actors' Equity will not let it ''appear to condone the casting of a Caucasian'' in the role of someone half Caucasian and half Asian. This denial of artistic freedom means that Pryce will be denied the sort of visa that enables a visitor to work temporarily in the United States.

Remember when liberalism had something to do with liberty? When liberals were strong for freedom to travel and against denials of visas?

If the union sticks to what it calls its ''moral position,'' -- it is wobbling -- the cost will include: a record $25 million advance ticket sale, 50 acting jobs (34 of them for minorities, most of them Asians -- isn't liberal compassion lovely?), 129 jobs for musicians, stagehands and others, and a payroll of $200,000 a week. Some union.

The union's weaselly position is the usual one of liberals running a racial spoils system. It says colorblind hiring is the correct principle, but in today's climate of racism, etc., and given the legacy of racism, etc., we cannot adhere to the correct principle until tomorrow. And tomorrow is always a day away.

With the pathetic earnestness of the ideologically blinkered, the union justifies its decision by citing a clause in the production contract affirming a ''policy of nondiscrimination.'' Such demented thinking results when language is reduced to applesauce. That has been done by dressing up reverse discrimination as ''affirmative action.''

When Justice Brennan resigned, there was much celebration of his ''legacy.'' The ''Miss Saigon'' scandal is part of it. He is particularly responsible for giving a constitutional imprimatur to the poison of ''progressive'' racism seeping through America's system.

People like the leaders of Actors' Equity, having been tutored into inanity by liberal jurisprudence, are too confused to have even an uneasy conscience when they say they are too gosh-darned moral to ''give away'' Pryce's role to Pryce.

He is understandably impatient with that way of talking about the role he created, artistically. Frank Rich, the New York Times theater critic, says Pryce's ''brilliant'' performance ''is as essential to 'Miss Saigon' as Joel Gray's was to 'Cabaret.' ''

But the union is nothing if not armed with the buzzwords of today's righteousness. It says that if Pryce were given permission to play the role, ''the Asian community'' would consider it a ''racial slur.'' You see, race-based decisions are acts of superior sensitivity.

Remember Yul Brynner playing the King of Siam in ''The King and I.'' Laurence Olivier playing Othello? Actors' Equity is now too moral to tolerate such casting.

Morgan Freeman, the black actor who starred in the films ''Glory'' and ''Driving Miss Daisy,'' recently played Petruchio in ''The Taming of the Shrew.'' Denzell Washington, another black star of ''Glory,'' is now playing the title role in ''Richard III.''

Actors' Equity has attacked the cardinal principle of theatrical ethics, the categorical obligation to present the best possible performance. It also has shown itself oblivious to the magic of theater, where actors, by their language and presence, entice the audience's imagination up from all considerations extraneous to the text and stagecraft. This is the supreme artistic as well as moral importance of ''non-traditional casting'' that casts a gifted black man as an English king.

Remember this name: Cameron Mackintosh. He is the British producer who, by standing up for artistic freedom and against today's trendy racism, told some American liberals that he will not be party to their traducing of this American (and, long ago, liberal) principle: It is wicked to allocate opportunity on the basis of race.