The Navy won a backroom congressional victory last week that allows it to get out from under laws that protect whales, dolphins and other marine mammals, but in the process it touched off what could be a bruising battle over the future of one of the nation's most popular environmental measures.

Environmental advocates and their congressional allies, including a handful of Republicans, are promising to mount a concerted effort next year to overturn the Navy provisions and to turn protection of the appealing and often endangered marine creatures into an election-year issue.

Advocates for the oceans say there is a great deal at stake: They call the changes the biggest "rollback" of the Marine Mammal Protection Act since it was passed 30 years ago, and predict they will result in the death of many more whales, dolphins and porpoises.

Apparently encouraged by its triumph, the Navy has served notice that it might try to modify a court agreement it accepted last month that sharply limits where it can deploy a new low-frequency sonar system designed to detect quiet diesel submarines. Environmentalists say the new sonar is so loud that it can harm and even kill noise-sensitive whales and dolphins.

The Navy got its way in the form of an amendment to the annual Defense Department authorization bill, which was sent Wednesday to President Bush. The long-sought provision essentially allows the Pentagon to exempt the Navy from marine mammal protections and sets out a looser definition of what constitutes illegal "harassment" of ocean mammals. The military has argued for some time that the law's requirements have limited its operations and technology in ways that endanger its forces by making it harder to track enemy submarines.

The legislative changes "will allow us to better provide the training that our young men and women in uniform deserve before being asked to sail into harm's way on our nation's behalf," said Navy spokesman Cappy Surette.

The use of the defense authorization bill as the vehicle for the Navy amendments invaded jealously guarded turf lines in Congress, raising the hackles of Sen. Olympia J. Snowe (R-Maine) in particular. She chairs the subcommittee that has jurisdiction over the marine mammal act, which is up for renewal next year.

Snowe said that the sponsors of the changes "disregarded our jurisdiction and previous work on the reauthorization of the Marine Mammal Protection Act, and they have seriously altered marine mammal policy in the United States." Many of those changes, she said, cause her "serious concern."

The Marine Mammal Protection Act was passed in 1972 to keep government and commercial interests from harming the declining populations of whales, dolphins and porpoises. The act is often credited with stabilizing some of those populations, and advocates are concerned that the progress will stop.

In a bid to build momentum for a review of the Defense-inspired changes, Sen. Susan Collins (R-Maine) sent a letter late last week to leaders of the Commerce Committee asking for a full review of the issue. She said she supports some changes to the law, but the Defense Department amendments "go beyond what is necessary to defend our nation and do not include adequate provisions to protect marine mammals, or indeed, even to know the impact of the provisions on marine mammals."

Gerald Leape, vice president of the National Environmental Trust, said the way the amendment went through -- with little public debate and a minimum of scientific input -- was particularly unfortunate.

"We and other groups will be taking on this fight with great vigor," he said. "They've created big holes in the [marine mammal act], and that has long been the backbone of the protection for these animals."

The Navy sought revisions in the act partially in response to its long-running legal battle with environmental groups trying to curtail deployment of a new low-frequency active sonar system. The Navy says that it needs the sonar to detect diesel submarines -- used by North Korea and others -- that sometimes cannot be traced with today's equipment, and that tests show the sonar is safe for marine mammals.

But environmental groups argue that the low-frequency sonar, which sends out underwater blasts of sound as loud as a jet engine, is likely to harm endangered whales and other sea mammals, and should only be used in selected areas. In recent years, several beachings of whales around the world have been linked by scientists to warships using mid-frequency sonar, and some believe the low-frequency version could be equally deadly.

Last month, the Navy agreed to a permanent injunction restricting deployment of the sonar, after a federal judge ruled that government agencies had improperly granted the necessary approvals.

Navy spokesman Surette said that after the congressional action, its officials are discussing with the Justice Department how it might reopen the case. "We are considering our appellate options, and these legislative decisions will factor into those potential courses of action," he said.

The changes to the law were crafted in a House-Senate conference committee that worked out the final language of the defense authorization, which also loosened some of the Pentagon's obligations under the Endangered Species Act. Military leaders have complained that large sections of training sites such as Camp Pendleton in California have become off-limits because they are home to endangered species, and the new legislation would ease the military's obligation to protect "critical habitat" for protected animals.

The House and Senate overwhelmingly approved the final language of the defense authorization bill, but some legislators said opposition to the environmental provisions was masked by the legislators' desire to support soldiers fighting in Iraq and Afghanistan.

"If there was an up-or-down vote on the changes alone, I don't think they would pass Congress," Collins said.

Sen. Carl M. Levin (D-Mich.) criticized the amendments Wednesday on the Senate floor, and said they need to be reviewed in the proper Senate committee. He also criticized the Bush administration for rejecting "every proposal that could have garnered broad bipartisan support in favor of an approach that would impose virtually no obligation at all on the Department of Defense to be environmentally responsible. I am concerned that this approach could result in real and unnecessary harm to marine mammals and a serious backlash against the Navy."

Joel Reynolds, senior attorney for Natural Resources Defense Council, said he did not think the amendment would immediately undermine the low-frequency sonar injunction inasmuch as the judge ruled that the government had broken a number of laws in approving the sonar deployment. But he also said that "any activity in the ocean that the military engages in will now be subject to less regulation as a result of these ill-advised exemptions."