"The fight is not over yet," said Sen. Arlen Specter, R-Pa., an opponent of the changes who is facing a tough re-election bid in November.

Congress reconvenes Sept. 7, but critics of the rules acknowledge a repeal is a long shot, given the threat of a veto by Bush.

Employers have sought the changes for decades, complaining the rules were ambiguous and out of date, and questioning why highly paid professionals should get overtime pay. Retailers, restaurants, insurance companies and others were getting hit with multimillion dollar lawsuits by workers claiming they were cheated out of overtime pay.

The Labor Department says the new rules provide clarity.

"Under the new rules, workers will know their overtime rights, employers will know their responsibilities and the department can more vigorously enforce these protections," Labor Secretary Elaine Chao said in a statement.

Labor unions say the new rules are intended to reduce employers' costs by cutting the number of workers who are eligible for overtime pay.

"The middle class is getting a gut punch on overtime," said Democratic Sen. Tom Harkin of Iowa, who has fought unsuccessfully to block the rules.

About 11.6 million workers receive overtime pay for working more than 40 hours a week, the Labor Department has said.

In all, about 115 million workers are covered by the overtime rules in the 1938 Fair Labor Standards Act. Estimates of how many workers will lose their overtime eligibility vary from 107,000 to 6 million. Calculations of workers who could become newly eligible range from very few to 1.3 million.