After nearly 25 years at Wal-Mart, Christine Kwapnoski makes more than $60,000 a year in a job she enjoys most days.

But Kwapnoski says she faced obstacles at Wal-Mart-owned Sam’s Club stores in both Missouri and California: Men making more than women and getting promoted faster.

She never heard a supervisor tell a man, as she says one told her, to “doll up” or “blow the cobwebs off” her make-up.

Once she got over the fear that she might be fired, she joined what has turned into the largest job discrimination lawsuit ever.

The 46-year-old single mother of two is one of the named plaintiffs in a suit that will be argued at the Supreme Court on Tuesday. At stake is whether the suit can go forward as a class action that could involve 500,000 to 1.6 million women, according to varying estimates, and potentially could cost the world’s largest retailer billions of dollars.

But the case’s potential importance issue goes well beyond the Wal-Mart dispute, as evidenced by more than two dozen briefs filed by business interests on Wal-Mart’s side, and civil rights, consumer and union groups on the other.

The question is crucial to the viability of discrimination claims, which become powerful vehicles to force change when they are presented together, instead of individually. Class actions increase pressure on businesses to settle suits because of the cost of defending them and the potential for very large judgments.

Columbia University law professor John Coffee said that the high court could bring a virtual end to employment discrimination class actions filed under Title VII of the Civil Rights Act of 1964, depending on how it decides the Wal-Mart case.

“Litigation brought by individuals under Title VII is just too costly,” Coffee said. “It’s either class action or nothing.”

Illustrating the value of class actions, Brad Seligman, the California-based lawyer who conceived of and filed the suit 10 years ago, said the average salary for a woman at Wal-Mart was $13,000, about $1,100 more than the average for a man, when the case began. “That’s hugely significant if you’re making $13,000 a year, but not enough to hire a lawyer and bring a case.”

The company has fought the suit every step of the way, Seligman said, because it is the “biggest litigation threat Wal-Mart has ever faced.”

A trial judge and the federal appeals court in San Francisco, over a fierce dissent, said the suit could go forward.

But Wal-Mart wants the high court to stop the suit in its tracks. The company argues it includes too many women with too many different positions in its 3,400 stores across the country. Wal-Mart says its policies prohibit discrimination and that most management decisions are made at the store and regional levels, not at its Bentonville, Ark., headquarters.

Theodore J. Boutrous, Wal-Mart’s California-based lawyer, said there is no evidence that women are poorly treated at Wal-Mart. “The evidence is the contrary of that,” Boutrous said.

The company is not conceding that any woman has faced discrimination, but says that if any allegations are proven, they are isolated. “People will make errors,” said Gisel Ruiz, Wal-Mart’s executive vice president for people, as the company calls its human resources unit. “People are people.”

Ruiz paints a very different picture of the opportunities offered women at Wal-Mart. She joined the company straight from college in 1992. “In less than four years, I went from an assistant manager trainee to running my own store,” she said. “I’m one of thousands of women who have had a positive experience at Wal-Mart.”

Kwapnoski, who works at the Sam’s Club in Concord, Calif., is one of two women who continue to work at Wal-Mart while playing a prominent role in the suit. The other is Betty Dukes, a greeter at the Walmart in Pittsburg, Calif.

“It’s very hard for anyone to understand how difficult that is and what courage that is,” Seligman said of Kwapnoski and Dukes. “They’re Public Enemy No. 1 at Wal-Mart and they are known for their involvement in this lawsuit. Nevertheless, they get and up and go to work every day.”

Kwapnoski didn’t want to discuss any issues she faces at work as a result of the suit.

She said she has seen some changes at Wal-Mart since the suit was filed in 2001. The company now posts all its openings electronically. “It does give people a better idea of what’s out there, but they still can be very easily passed over.” she said. “But before you didn’t even know the position was open.”

The suit, citing what are now dated figures from 2001, contends that women are grossly underrepresented among managers, holding just 14 percent of store manager positions compared with more than 80 percent of lower-ranking supervisory jobs that are paid by the hour. Wal-Mart responds that women in its retail stores made up two-thirds of all employees and two-thirds of all managers in 2001.

Kwapnoski said she and a lot of women were promoted into management just after the suit was filed, although she has had only a couple of pay increases in the nine years since. She is the assistant manager in her store’s groceries and produce sections.

Now, she said, promotions are back to the way they were before, favoring men over women.

She said she’s hoping the long-running court fight will force Wal-Mart to recognize that, stories like Ruiz’s aside, women are not valued as much as men are and that her bosses will begin to “make sure that good men and good women are being promoted, not just men.”