WASHINGTON — This year marks the 15th anniversary of the landmark Family and Medical Leave Act, which made it possible for many workers to take unpaid job-protected time off to care for their newborn children or sick relatives. But instead of celebrating, workers' rights advocates and the Bush administration are battling over what would be the most sweeping revisions ever to the law.

Under proposals being considered by the Labor Department, workers would have to tell their bosses in advance when they take nonemergency leave, instead of being able to wait until two days after they left. They would have to undergo "fitness-for-duty" evaluations if they took intermittent leave for medical reasons and wanted to return to physically demanding jobs. To prove that they had a "serious health condition," they would have to visit a healthcare provider at least twice within a month of falling ill. What's more, employers would have the right to contact healthcare providers who authorized leave.

These and other proposed changes have set off a fierce debate. More than 4,000 comments were submitted to the Labor Department as of yesterday, the deadline for the public to weigh in. They came from labor unions, religious organizations, women's rights groups, small and large business owners and employees across the country.

There were queries such as these, from Rita Palmer at the Planet Hollywood Resort and Casino in Las Vegas: "What alternative does an employer have when an employee must leave the country for an ill parent? What if abuse is suspected? We had an employee leave for Italy every summer and submitted a WH-380 from the mother's attending physician."

And pleas, such as these, from Richard Kirk, chief steward of the Sacramento Area Local American Postal Workers Union: "Given the outsourcing and the loss of jobs overseas, Americans must hold on to what we have today, for fear of losing it tomorrow. FMLA is but one way to ensure employees have a 'balanced' playing field."

At the crux of it all, said advocates of the FMLA and of its revisions, is the uncertainty fostered by the wobbly economy. Workers want assurances that their jobs will be safe even when they have family or medical emergencies. Businesses want to make sure they are operating efficiently, getting the most for their money.

"There's a lot of interest in getting the debate and discussion going on how to make things better for everyone, whether it be employers, employees or families," said Cara Welch, director of Public Policy for WorldatWork, an association of personnel professionals. "They are highlighted now with the focus on a presidential election and with an economy that is a little bit more on a downward slope."

Any changes would have widespread impact. In 2005, the last year for which the Labor Department has data, nearly 7 million people used the FMLA, which allows for as much as 12 weeks of unpaid leave. Workers rely on the FMLA, which applies to companies with at least 50 employees, because many companies do not offer paid sick leave or disability coverage.

There is no federal law requiring paid sick leave. The D.C. Council voted last month to make the District the second city in the nation to mandate some paid sick leave. San Francisco is the other city with such a law. According to the Bureau of Labor Statistics, 57 percent of workers in private industry had access to paid sick leave in 2006.

Some changes have been made. In January, Congress passed and President Bush signed an act that allowed military families to take extended leave to care for an injured service member.

That has buoyed other efforts. Democrats in Congress have introduced a number of bills that would guarantee more time off when family needs arise. The Healthy Families Act, for one, would give workers seven days of paid sick leave to care for their own medical needs as well as those of a family member. The Balancing Act would provide paid family medical leave, benefits for part-time workers and time off for activities that require parental involvement, such as medical appointments.

Although Democrats control Congress, it is unclear how far such bills would go. Even if they made it through the House and Senate this session, getting past a Republican in the White House could prove difficult, according to a Democratic staff member in the House and some FMLA advocates.

"There's growing support among policymakers and growing understanding for the importance of these measures," said Vicky Lovell, director of employment and work/life programs at the Institute for Women's Policy Research, a Washington nonprofit organization promoting women's issues.

But, she added, "Once a new administration is in place, that would definitely add a lot of uncertainty."