WASHINGTON -- President Bush's vision for protecting two vast areas of the Pacific Ocean from fishing and mineral exploitation, a move that would constitute a major expansion of his environmental legacy, is running into dogged resistance both inside and outside the White House and has placed his wife and his vice president on opposite sides of the issue.

With less than three months before Mr. Bush's term ends, his top deputies are scrambling to try to execute a plan that would shield some of the world's most diverse underwater ecosystems. The original plan, which included four potential "marine monuments" and was well-received by environmentalists, has already been scaled back.

Vice President Dick Cheney and some officials in the Commonwealth of the Northern Mariana Islands have argued that the plan could hurt the region's economy by barring fishing and energy exploration. First lady Laura Bush, along with a number of scientists and environmental advocates, has countered that preserving the region's natural attributes would attract tourism and burnish the president's record for history.

Mrs. Bush has asked for two briefings on the issue from White House staff members, and her aides have conferred with scientists who support the two designations.

"It's hard, but it should be," said James L. Connaughton, who chairs the White House Council on Environmental Quality and just returned from an overseas listening tour on the proposal. "These are big, consequential, national decisions that have international ramifications."

While environmental groups have pilloried Mr. Bush over his approaches to climate change, forest management and air pollution, many marine experts give him credit for his ocean policies.

In 2006, he designated the nearly 140,000-square-mile Papahanaumokuakea Marine National Monument in the northwestern Hawaiian Islands, creating what at the time was the world's largest protected marine area. Scientists have advocated designating more such areas to protect them from the effects of overfishing, pollution and global warming, which are degrading oceans worldwide.

"There's pretty strong evidence that everyone will benefit from the establishment of no-take reserves," said Jane Lubchenco, an Oregon State University professor of marine biology, adding that fish populations rebound both within the protected reserves and in nearby fishing grounds.

"The administration made a major step forward in designating the Papahanaumokuakea National Monument," she said "but that one alone is not enough to protect the full range of places and habitats and species that need to be protected. It will be part of [Bush's] legacy, but his ocean and environmental legacy could be much, much more."

Researchers and activists welcomed Mr. Bush's August memorandum asking Mr. Connaughton and the secretaries of defense, commerce and the interior to assess the two "marine conservation management areas" he might establish before leaving office.

One, in the central Pacific, would encompass an area known as the Line Islands and stretch about 2,000 miles from the Johnston Atoll to the Rose Atoll. The memo described the area as "isolated from population centers, mostly uninhabited" and supporting "endemic, depleted, migratory, endangered and threatened species of fish, giant clams, crabs, marine mammals, sea turtles, seabirds, migratory shorebirds and corals that are rapidly vanishing elsewhere in the world."

The other area, in the western Pacific, would include the waters around two northern Mariana Islands and the 6.8-mile-deep Mariana Trench, the deepest ocean canyon in the world.

Both regions are treasure troves of biodiversity: Kingman Reef and other islands in the central Pacific area teem with sharks and other top predators; the Mariana Trench and its nearby islands are home to several species of rare beaked whales and the Micronesian megapode -- an endangered bird that uses the heat from volcanic vents to incubate its eggs -- as well as to mud volcanoes, pools of boiling sulfur and the greatest microbial diversity on Earth.

No one questions the ecological, biological and geological value of these sites, but supporters of protecting them -- including Mr. Connaughton and advocacy organizations such as the Pew Environment Group -- have faced serious opposition in convincing several key White House officials of the value of broad "no-take" reserves.

Mr. Bush initially explored the idea of establishing other protected areas closer to U.S. shores, including one off the southeastern coast near a group of deep-sea corals and another in the Gulf of Mexico. After commercial and recreational fishing interests and oil companies objected, the administration decided to pursue existing resource-management plans in those areas instead.

Despite the islands' distance from the continental United States, the proposal to designate an area around the Northern Marianas -- a U.S. commonwealth between Japan and Guam -- has sparked considerable debate. Mr. Cheney and National Economic Council Director Keith Hennessey have questioned the impact on the region's economy, a concern some local officials also raised.