The "Coal Miner's Daughter" showed up here today in a new role--lobbyist in the state-wide campaign against uranium mining.

"I think this is the most important issue facing Virginia," declared Sissy Spacek, the effervescent, ponytailed actress whose appearance startled state legislators more accustomed to lobbyists with fat cigars and pin-striped suits.

"I thought, well, Sissy Spacek, I must be seeing things," said an astonished Del. Willard R. Finney (D-Rocky Mount) who returned from lunch to find Spacek, her husband and a group of citizens from southern Virginia lounging outside his office.

Spacek jumped up, shook his hand and made a brief pitch on behalf of what she said was her first public cause.

"We moved here from L.A. for clean air and clean water," said Spacek, who lives on a farm near Charlottesville with her husband, director Jack Fisk, and their 6-month-old daughter, Schuyler. "It's alarming to us."

Mining of radioactive uranium is expected to be one of the hot topics when the General Assembly convenes here Wednesday. The Marline Oil Co. and Union Carbide Corp. are backing a bill they hope will eventually allow them to open a $200 million mining operation in the tobacco fields of southern Virginia's Pittsylvania County, near the North Carolina border.

Environmentalists, worried about radioactive contamination of the region's water supplies, are battling the companies and last month asked the 33-year-old actress to join their cause. Spacek agreed because she says she's worried about her daughter's future, even though she said she wasn't the world's foremost expert on the subject.

"The only thing I knew about uranium was from an article I read in the Texas Monthly a couple of years ago about a doctor who was trying to get rid of his 7-year-old son by sticking a little piece of uranium under his pillow and making him wear a little piece of uranium around his neck," she said.

But since then, she said, she's studied up on the matter, particularly on the links between radioactivity and cancer.

"It blows my mind how many people have cancer," she said, shaking her head.

"Last year my mother died of cancer, my brother died of leukemia at 19 and my other brother has had cancer," Spacek said. "I'm not an expert but I do know there's a great deal of scientific research linking radiation to cancer."

Spacek's competency as a scientific witness was challenged by a Union Carbide official, who showed up at her morning news conference and later called her comments an "alarmist, emotional type of testimony."

"It's a ploy," said Dudley Blancke, a Union Carbide spokesman. "Everybody has a right to their opinion, but a lot of her statements were misleading . . . the general public has never been shown to be harmed by uranium mining and milling."

But Spacek, who once starred in the horror classic "Carrie" about a teen-ager with supernatural powers, joked that she's even prepared to use Carrie's telekinetic talents to impress recalcitrant legislators.

"You want to see me level that gymnasium over there?" she asked, narrowing her green eyes.