People used to worry about television de-sensitizing us to violence. Now the big concern is that it may be desensitizing us to truth, playing such dangerous games with fact and fancy as to blur the distinction between the two.

"The Trial of Lee Harvey Oswald," a four-hour ABC TV movie airing in two parts - tonight and Sunday night at 9 o'clock on Channel 7 - is beyond question tasteless and reprehensible as a piece of entertainment, so it's only natural to go on to the question of whether the party-game it plays with tragic history represents a threat to the national mental health as well.

TV news is drifting further into showbiz. TV movies and plays are increasingly going the "docu-drama" route in which real people (Caryl Chessman and Karen Ann Quinlan in recent weeks) are dropped into formula potboiler stories - entertaining ordeals. Journalism and escapism aren't just cross-pollinating in television they are cross-polluting.

The "trial" of Lee Harvey Oswald for the assassination of President John F. Kennedy never occured, of course, since Oswald himself was murdered, and no real purpose is served by supposing that it did. But by dropping real names as loudly was possible into a script, inevitable if unsavory impact can be added to a story that in this case would have none without them.

At least the creators of "Washington: Behind Closed Doors" had an imposing point to make with their fictionalized Watergate saga, and it was presented as fiction, not fact.

But ABC has bragged of the "Oswald" show that it is "based on historical fact, not speculation or rumor." When, during filming in Dallas Last summer, supervising producer Lawrence Schiller was criticized for inaccuracies in the re-staging of the assassination, he responded by saying, "We're here to recreate it emotionally," and, "We should not allow the enormity of the event to overshadow the simplicity of the act."

Schiller, the Dino de Laurentiis of the graveyard, is also the showoman who bought the rights to Gary Gilmore's life before Gilmore's execution. He goes beyond everyday vulgarity with "Oswald." The jury's verdict in the mock trial has been kept a cliffhanging secret, and ABC will invite viewers to write in their opinions for a poll whose results will be announced on the Oct. 14 "Good Morning, America" show.

Even from the network that gave us "Let's Make A Deal" and "Soap," this seems an incredibly smarmy project. It certainly refutes those who said that Paddy Chayefsky's satire of television greed, "Network," was too farfetched to be plausible. Schiller and ABC have made Chayefsky's hysteria look almost namby-pamby.

For what it's worth. David Greene's direction of "Oswald," at least on the first two hours previewed, gives it an unmistakable shimmer of urgency. The re-enactment of the assassination does command one's attention - party because it seems so visually disconcerting to see Dealey Plaza and the Texas School Book Depository in color - but soon we realize that the creators of this sorry charade have by no means justified bringing the painful subject up again.

A the point in the story where Oswald would have been shot by Jack Ruby, Greene cuts to a blinding glare that is supposed to separate the history from the fantasy. There is reason to believe that so far as matters of truth are concerned, however, television is becoming one continuous blinding glare itself.