The proposal to drill for oil in an Alaska wildlife refuge will create the most environmentally friendly oil field in the world, Interior Secretary Gale Norton said Tuesday — a claim disputed by drilling opponents.

They said the legislation before Congress could allow oil companies to skirt some existing environmental safeguards while weakening the ability to challenge oil leases in court.

The standoff in the Senate over drilling in the Arctic National Wildlife Refuge in northeastern Alaska was expected to reach a climax Wednesday with a critical vote on whether the measure should be included in a massive defense spending bill.

The outcome could be determined by one or two votes.

Drilling opponents have argued for years that opening the refuge's 1.5 million-acre coastal plain to oil companies would threaten caribou calving areas and adversely disturb a pristine area that is home to polar bears, musk oxen and, during certain times of the year, millions of migratory birds.

Those who have sought access to what may be 10 billion barrels of crude oil maintain that a string of safeguards put into the legislation — from limiting the "footprint" of development to requiring use of the most modern drilling technology — would protect the wildlife.

"The requirements in the legislation would be very strict," Norton said in an interview Tuesday, calling the environmental standards for the refuge "more stringent than is applied in other oil and gas producing areas." She said the bill requires her to ensure leasing plans are "environmentally sound" with "no significant adverse impact" to wildlife or their habitat.

Sen. Lisa Murkowski, R-Alaska, said the bill "lets us address the environmental issues" and provides safeguards by requiring advanced technology. "We're not talking about opening the entire 1.5 million acres. ... We're asking for permission to explore and drill in an area not to exceed 2,000 acres," Murkowski said on the Senate floor.

But environmentalists called the 2,000-acre limit "a sham" that does not reflect the fact that the bill would allow drilling activities in all but 45,000 acres of the coastal plain, crating a "spiderweb" of development.

The legislation would require Norton to offer oil leases covering at least 200,000 acres within 22 months and a second lease package in 2010. The 2,000-acre footprint limit applies, according to the bill, only to support facilities, airstrips, areas covered by gravel berms and the pipeline support piers where they actually touch the ground. Environmentalists note that actual pipelines, which are off the ground, are not considered part of the footprint, thought they argued the pipes would have an impact on the wild nature of the region.

Sen. Maria Cantwell, D-Wash., a leading opponents of ANWR's development, called the legislation "a back door that circumvents" existing environmental laws. "It creates ill defined environmental standards ... and cuts off the secretary's ability to protect environmentally sensitive areas," said Cantwell, alluding to the 45,000-acre limit on special protection areas.

The bill also would require court appeals on lease decisions to be filed within 90 days.

It also requires use of a 1987 environmental analysis as the basis for setting standards. Environmentalists complained the National Environmental Policy Act analysis is out of date and ignores more recent findings, including a 2002 Interior Department report that concluded oil drilling in the refuge would pose substantial risk to calving grounds for caribou.

Norton said she believes the proposed standards "are very strong" and she anticipates "doing very extensive environmental work" beyond the 1987 analysis. She said the 2002 Interior Department report on drilling impacts on wildlife did not take into account the mitigation and restrictions required in the legislation, including the 2,000-acre footprint limit.

"They went back and analyzed scenarios with the (new) restrictions. They determined there would be very little impact on caribou population," said Norton.