WASHINGTON – Paychecks could surge or shrink for a few or for millions of workers across the country starting today, when sweeping changes to the nation’s overtime pay rules take effect.

There is little agreement by the Bush administration, employer groups, labor experts, and others on how many workers will gain or lose the right to overtime pay under the new rules in the Fair Labor Standards Act.

“To be candid, no one knows,” said Jerry Hunter, a labor lawyer at Bryan Cave LLP in St. Louis and former general counsel of the National Labor Relations Board during the first Bush administration.

Employers have sought changes for decades, complaining the regulations were ambiguous and out of date and questioning why highly paid professionals should get overtime pay. Labor unions, however, say the new rules are intended to reduce employers’ costs by cutting the number of workers who are eligible for overtime pay.

Estimates of how many workers will lose their overtime eligibility range from 107,000 to 6 million. Workers who could become newly eligible range from very few to 1.3 million.

“Not only is the Labor Department unsure, but a lot of people in a lot of industries are unsure,” Mr. Hunter said. “This is all very fluid right now.”

The major overhaul, the first in more than half a century, is aimed at mostly white-collar workers. The Labor Department says manual laborers and other blue-collar workers will not be affected.

The new rules are intended to limit workers’ multimillion-dollar lawsuits, many of them successful, claiming they were cheated out of overtime pay for working more than 40 hours a week.

Retailers, restaurants, insurance firms, and banks have been targets, and jobs in those places are generally exempted from overtime in the new rules. They include chefs, pharmacists, funeral directors, embalmers, journalists, insurance claims adjusters, low- and midlevel bank managers, and dental hygienists.

Whether the new rules will reduce litigation is questionable, experts said. Lawyers representing workers have found the suits lucrative.

“This has become a very big area of plaintiffs’ employment law, and it is not simply going to go away because of these new regulations,” said Bill Schurgin, a labor lawyer in the Chicago office of Seyfarth Shaw.

Critics say the changes will eliminate overtime for millions of middleclass Americans struggling in a weak job market.

“These are drastic changes that will hurt working families,” said Karen Nussbaum, the executive director of Working America, an AFL-CIO organization created for workers unable to join unions. The AFL-CIO is holding a protest outside the Labor Department today.

Labor Secretary Elaine Chao has created a task force that will be “looking very closely and critically at any reclassifications that result in workers losing their overtime status,” said a deputy secretary, Steven Law.

No new funds have been added, but the department’s Wage and Hour Division “will be very, very carefully monitoring and following up with enforcement,” especially in high-violation industries, he said. The department won $212 million in back wages for overtime violations in 2003, a 21% increase.

At Denver Water, a public utility, none of the 1,050 employees will be reclassified, said benefits manager Jim Crockett. “We were in compliance before, and when I analyzed the jobs for the new rules, it came up that no changes were necessary,” he said.

About 107,000 white-collar workers now eligible for overtime pay who earn $100,000 or more annually could lose it under the new rules, the Labor Department said.

About 1.3 million workers, mostly low- and midlevel managers at stores and restaurants, who earn less than $23,660 a year will be newly eligible. However, employers can avoid paying them overtime by raising their salaries, so critics say far fewer will benefit from overtime.

For white-collar workers who fall between those salary levels, their overtime status depends on their job duties and experience. The rules revamp the definitions of professional, administrative, and executive employees, called “duties tests,” that are used to determine eligibility.