Despite intense lobbying over oil drilling in an Arctic wildlife refuge, big oil companies largely have kept silent, their attention on exploration elsewhere.

The oil industry long has sought to drill for the billions of barrels of oil beneath the refuge in Alaska's northeastern corner. But drilling supporters complain privately that large oil companies have not pushed aggressively to open the site to development.

A Senate vote was expected Thursday on the future of the Arctic National Wildlife Refuge.

"We have worked harder than anyone else ... we've done as much as we can," insisted Red Cavaney, president of the American Petroleum Institute, the companies' trade association. The refuge "is a very valuable resource," he said Wednesday.

But an industry insider, speaking on condition of anonymity, acknowledged that for many of the large companies, the refuge is only one place to find oil and that there are many fields around the world where development would come more cheaply and with less political trouble.

In setting priorities, many companies have expressed more interest in other pro-industry provisions in the energy bill; for example, curtailing government regulations on how gasoline is blended for pollution control.

Still, the refuge has attracted lobbying for months as Republicans searched for votes to overcome a threatened filibuster from Democrats and other opponents intent on keeping a drilling amendment out of the energy legislation.

Lobbyists for the Teamsters, working Senate corridors, tout the jobs that drilling would produce. Rival Alaska native groups have flown in from the North Slope to argue both for and against drilling. The state of Alaska, which would get half the royalties from oil development, has poured millions of dollars into the pro-drilling effort. Environmentalists have bombarded Senate offices with e-mails challenging claims by the pro-drilling side.

Groups as diverse as military veterans and leading Jewish organizations have come to Capitol Hill to argue that the refuge's oil will reduce America's dependence on oil imported from the volatile Middle East.

In response, anti-drilling forces produced experts such as former CIA Director James Woolsey, who told reporters this week that drilling in the refuge would do little to enhance national energy security because America always will need Mideast oil.

The most intense pro-drilling campaign has come from Alaska itself. Not only are the state's two senators leading the push for drilling, but the Alaska Legislature has contributed nearly $5 million to the effort over the past two years.

Much of that money funds Arctic Power, a lobbying group whose sole purpose is to open the refuge.

With President Bush a strong supporter of drilling and the House already on record in support of opening the refuge, prospects are as good as they ever have been, says Roger Herrera, Arctic Power's lobbyist in Washington.

For weeks, Arctic Power has assembled a "war room" a few blocks from the Capitol from which they carry on the fight. It was a busy place Wednesday.

"We've always had a very tough vote... We've always been the underdog," said Tara Sweeney, a member of the Inupiat tribe, as she worked out of those headquarters. Sweeney has made repeated 10-hour trips from Alaska to Washington in recent months to argue for drilling. Her mother had done the same in years past.

She tells senators that drilling in the refuge will determine the future of her tribe. The Inupiat own part of the land on the coastal plain of the refuge and stand to benefit greatly if the area is allowed to be developed.

Also lobbying Congress are representatives of a second group of Alaska natives, the Gwich'in tribe. They argue that oil drilling will threaten the Porcupine caribou, on which they depend.

"Oil development is a threat to our very existence," said Faith Gemmill of the Gwich'in Steering Committee. Like Sweeney, she too has made more than a few trips to Washington to tell her story, hoping it will sway a senator to her side.