Karla Cowell had always believed that "abortion was an unforgivable act and that women who had abortions were committing a sin."

Then, just out of college and drifting along in a temporary secretarial job, she became pregnant.

"For 22 years I had believed that abortion was just not an answer, and yet on the day I personally was faced with the situation, it became the answer," she said. "The lesson for me was that never can a person possibly know what their actions will be in a situation unless faced with the situation."

Cowell, now 25 and a research associate at Gallaudet College here, never told her parents she had the abortion or talked about it with friends. Now, she is speaking about it publicly as part of a nationwide campaign by abortion rights advocates that comes to Washington today.

Called "Abortion Rights: Silent No More," the "speak-out" campaign is designed to recapture momentum from the antiabortion movement and to combat antiabortion efforts such as "The Silent Scream," the controversial videotape of a first-trimester abortion that describes the fetus writhing in fear and pain during the procedure.

After events in nearly every state, the "Silent No More" campaign opens in Washington today with a public reading of letters from women and men describing their experiences with both legal and illegal abortions, and personal accounts from Cowell and women and men from all 50 states.

The event is set from 7 a.m. to midnight at Western Plaza, 14th Street and Pennsylvania Avenue.

Tomorrow, the state representatives will deliver to Congress some of the 45,000 letters the National Abortion Rights Action League (NARAL) has gathered.

"By the end of January when President Reagan addressed a record crowd of more than 71,500 antiabortion demonstrators at the annual March for Life , the focus of the debate around abortion had really . . . focused on the fetus," said Nanette Falkenberg, executive director of the abortion rights league. "We needed to bring back into the debate the real world of men and women. We needed to remind a whole generation of people what it was like before abortion was legal."

Scheduled speakers today include Jack Loegering, 65, a retired high school teacher from Prior Lake, Minn., whose sister died after an illegal abortion, and Sherry Matulis, a 53-year-old mother of five and grandmother of three who had an illegal abortion 30 years ago after she was raped.

"I resent more than any words can say what I had to endure to terminate an unbearable pregnancy," Matulis wrote in a letter released by the league. "But I resent even more the idea that any woman should, for any reason, ever again be forced to endure the same."

The speak-out revives the tactic of publicizing the horrors of illegal abortions through personal stories that organizers say was effective in the state-by-state battle to liberalize abortion laws before the Supreme Court's 1973 Roe v. Wade decision legalizing abortion.

"This is is our strongest suit, and we haven't gone with it in a long time," Falkenberg said."

Antiabortion forces are countering with a press conference Tuesday at which representatives of Women Exploited by Abortion (WEBA), an antiabortion group of women who have had abortions, will discuss their own experiences.

"NARAL purports to have testimony from women who have had abortions and just think it's a great way to spend your Saturday afternoon," said Patti Haywood McKinney, a founder of WEBA. "Perhaps there are some women who through some coping mechanism are able to escape the intense guilt and grief that many of us suffered."

But, she said, "The vast majority of women with whom I've counseled consider it a totally negative experience. They wish that they could undo what they did. They long for that child that they threw away and they feel intense guilt because their own ignorance or their own irresponsibility cost the life of their own child.