Keri Gorder, until recently the manager of a hair salon in Great Falls, Mont., said she was surprised last month by a document that her company wanted stylists to sign.

Ms. Gorder said the salon’s parent company, the Regis Corporation, had urged the four stylists at her salon, Cost Cutters, to sign a document that would seemingly nullify any future support they showed for unionization.

Labor leaders in Montana accuse the company of seeking to take away the stylists’ right to form a union. But Regis says the document merely seeks to ensure that workers choose unions through a secret-ballot election — at a time when unions are pushing legislation in Congress that would make it easy to bypass secret ballots.

The document the stylists at several Montana salons were urged to sign said they were agreeing to revoke any future signature they put on a pro-union card that could be counted as showing support for unionizing.

“I thought it was taking our right away before we ever exercised that right,” Ms. Gorder said.

She said her area supervisor had pressured the stylists to sign the cards. “The area supervisor said, ‘I would do what the company wants you to do,’ ” Ms. Gorder said, adding that she quit her job this month because of her dismay over the situation.

Soon she informed labor leaders about the document, and now they are threatening to picket the salon and hand out pro-union fliers.

“It’s the craziest thing I’ve ever seen,” said Ole Stimac, president of the Central Montana Central Labor Council. “I’ve never seen anything where you sign away your rights for eternity to unionize.”

Regis executives said they had distributed the document out of concern that Congress would enact legislation backed by labor that would require employers to recognize a union as soon as a majority of workers signed pro-union cards, without holding a secret-ballot election.

Paul Finkelstein, chief executive at Regis, the nation’s largest hair salon company, said many employees signed such pro-union cards without understanding that it could commit them to joining a union. Mr. Finkelstein said the company’s focus groups showed that employees overwhelmingly favored using secret ballots to decide whether to join a union.

The document the hair stylists were asked to sign, titled Protection of Secret Vote Agreement, said, “In order to preserve my right to a secret-ballot election, and for my own protection, I knowingly and without restraint and free from coercion sign this agreement revoking and nullifying any union authorization card I may execute in the future.”

Mr. Finkelstein said the document was intended to ensure that the employees’ cards were never counted to show majority support for a union — in case Congress someday enacted the union-card legislation.

“The sole issue is that our people want to use a secret ballot,” he said, asserting that union organizers often manipulate workers into signing pro-union cards, known as authorization cards.

Mr. Finkelstein added: “We’re not threatening people, ‘You’d better sign.’ It’s totally voluntary.”

William B. Gould IV, a Stanford law professor and former chairman of the National Labor Relations Board, said, “It seems like a modernized version of the old yellow dog contract,” a provision, now illegal, that many employers used to push workers to sign, pledging not to join a union as a condition of employment.

Assessing the salon document, Mr. Gould said, “I think it’s illegal because an authorization card is the principal vehicle unions use to organize the unorganized.”

Under current law, at least 30 percent of a workplace’s employees must sign cards to lead to a secret-ballot election. Mr. Gould said that under the Regis document, cards signed to seek a secret ballot would automatically be revoked.