Three of America's premier actresses testified on Capitol Hill yesterday about the plight of the American farmer, in an orchestrated event combining just the right amount of politics, pathos and celebrity, and once again blurring that Washington line between reality and celluloid.

Jane Fonda, Jessica Lange and Sissy Spacek, stars of three recent films about strong rural women battling adversity, poured out their hearts and even some tears before a Democratic task force on agriculture at a hearing that otherwise would have been half empty.

"One camera crew at a time!" yelled a press aide who was substituting as something of a traffic cop outside the hearing room.

Lucky visitors could catch a glimpse of the back of Lange's royal blue suede tunic or the top of Jane Fonda's head. Stargazers were standing on chairs, and the press outnumbered members of Congress 5 to 1. The Democratic sponsors made no bones about their play for publicity on the farm issue, two months after President Reagan vetoed the farm emergency credit bill. They couldn't have asked for a better performance.

"It is heartbreaking to witness their anguish as they watch their lives being stripped away," Lange said of the farmers as she choked back tears.

The cameras clicked unmercifully through her every gasp, sniffle and silence. Soon Fonda joined in, delicately wiping her eyes as Lange spoke. The cameras went nuts. The pauses in the testimony were anxiety-provoking to those watching, as both actresses seemed on the verge of breaking into sobs.

"We thank you for your eloquence and your empathy," Rep. Tom Daschle (D-S.D.), the panel chairman, consoled.

Lange, who lives on a farm in New Mexico, starred in "Country," Spacek produced and starred in "The River," and Fonda developed and played the star role in the television film "The Dollmaker" -- all about the stresses facing farm families.

While Daschle said "our purpose is not political," the dozen members of the House Democratic Caucus Task Force on Agriculture produced an event with partisan overtones. No Republican members were invited, and it was cosponsored by the Democratic Congressional Campaign Committee, the party's House fund-raising organization.

All three actresses decried the farm policies of the Reagan administration as uncaring and insensitive to rural America's anguish.

Lange said she had spent "countless hours" in recent years talking with farmers about the effects of low crop prices and plummeting land values, which have brought on the worst farm financial crisis in decades.

"They are being made to feel and made to believe they have failed," she told the panel in a breaking voice. "Failed their families, their heritage, their country, and they have failed their land."

Spacek, who lives on a farm near Charlottesville, Va., said she feared that "our largest and most vital industry is disintegrating. It is not the marginal producers, speculators or bad managers that are being squeezed out, but the solid core of our agriculture which is threatened."

Fonda accused Reagan of practicing "a double standard" that offers more in subsidies to defense contractors than to farmers, and providing "loopholes wide enough to drive a tractor through for Beverly Hills attorneys who farm for tax breaks, but want to take incentives away from those who farm for a livelihood.

"The reason we are here is to underscore the gravity of the crisis that is leading to the bankruptcy, humiliation and banishment of farmers from their lands at a rate not seen since my father made 'The Grapes of Wrath,' " she said, referring to Henry Fonda's role in that film about the trials of a farm family in the Dust Bowl of the 1930s.

Daschle defended the event against GOP criticism, saying the actresses "do not pretend to be farm experts because they have each rendered a sensitive portrayal of farm life on film, but they do understand what so many of our farm families are going through," and adding that they were simply trying to help "educate America . . .

"To those who may believe that this whole issue has blown over," he said, "our presence here this morning is to emphatically say no . . . In fact, in some ways it's blowing even harder than it has been in the past."