Actor Charlton Heston, a former president of the Screen Actors Guild, has become embroiled in an increasingly bitter confrontation with the current SAG president, outspoken television star Ed Asner.

At issue is whether SAG will merge with several other craft unions, but the battle has taken on both personal and political overtones. Actors have chosen camps and rallied behind one or the other leader. Among those in agreement with Heston (anti-merger) are Irene Dunne, James Stewart, Robert Conrad, Dean Jagger, Mike Connors and Clint Eastwood. On the other side (pro-merger), Martin Sheen, for one, has come out in support of Asner, but the Screen Actors Guild, annoyed by the attention paid to which star is supporting which star, won't talk names. "The key is that the membership elected Asner president," said SAG information director Kim Fellner. "They voted him in knowing full well that he was a man who has opinions . . . We're really tired of headlines that say, 'Moses meets Lou Grant.' "

Heston said in a phone interview yesterday that he was drafted for the role of spokesman because of his "access to a public forum." Speaking to a group of about 250 actors Sunday night in Los Angeles, Heston said, however, that he would not support a drive to oust Asner, but would wait to see how Asner reacts to "the firestorm of protest that has sprung up."

Adding to the controversy is Asner's statement last week that he and other actors would personally send $25,000 in medical supplies to guerrillas fighting the government of El Salvador.

"I was asked to speak out on these issues--before I was even aware of them--by members of the guild and members of the guild board," said Heston, reached by phone yesterday in Los Angeles. "Obviously, I was asked because they'll bring television cameras over to my house . . . and it was thought that I would know what I was talking about." Since then, "I'm doing three or four television shows a day," he said.

Heston, an honorary (and non-dues-paying) life member of the guild, added, "I guess I'm paying my dues.".

Heston, a friend of President Reagan's, was irked by television reports that left the impression Asner was supporting Salvadoran guerrillas in his role as SAG president rather than as a private individual. "He was grossly negligent in his responsibility to make that clear," said Heston.

Heston also told Asner supporters on Sunday night that Asner called him "a name I cannot repeat with the television cameras running, and three witnesses have told me so." The incident occurred outside the SAG headquarters a couple of weeks ago where Heston and a group of actors were picketing. Asner came down to talk with them, a move that Heston commended. "I thought it was courageous . . . " Heston said yesterday. "He seemed to endorse our picketing as democratic." But then, said Heston, "he called me a c--------- . . . He wasn't speaking to me. But that's what he said . . . And apparently he said it well within microphones."

Heston sent Asner a letter on Feb. 11, which read in part, "The attack on me is meaningless. I'm only sorry you can't understand how deeply demeaning this is to you, your office, and our guild. It's as permanent and repellent a stain on a man as Khrushchev, drunk, pounding his shoe on the table at the U.N."

"His response was rather curious," said Heston, laughing. "He said, 'I will not dignify his letter with a response.' "

Asked whether Asner really did say it, Fellner replied, "People say things to their friends all the time. Whether or not it happened is irrelevant . . . I think Ed's response is the proper response. There are many more critical issues."

The 55,000-member Screen Actors Guild is expected to vote on whether to merge with the Screen Extras Guild and the American Federation of Television and Radio Artists.

Heston said the guild should "avoid mergers that will multiply the number of unemployed actors when production of motion picture films is at an all-time low . . . We have problems enough trying to take care of our own members. We just raised dues."