WASHINGTON — The U.S. Senate passed a budget resolution Thursday that could provide Alaska's congressional delegation its best shot in four decades to open part of the Arctic National Wildlife Refuge to oil and gas drilling.

The Senate voted 51-49 to pass the budget resolution, along party lines. Republicans defeated a Democratic amendment to strip the ANWR-allowing provision from the budget resolution, by a vote of 52-48.

It's not a slam-dunk yet. But Alaska Sen. Lisa Murkowski has been granted the opportunity to attach an ANWR drilling provision to a tax reform bill that is considered "must-pass" legislation if Republicans want to retain their control of Congress in 2018. That bill — known as "budget reconciliation" — will only require a simple 51-senator majority vote. House and Senate leaders have said they hope to pass tax legislation before the end of this year, though that may be an overly optimistic timeline.

The budget resolution passed Thursday instructs the Energy and Natural Resources Committee to write legislation that raises $1 billion in federal revenue. The House passed a budget resolution earlier this month with a similar provision.

Opponents of Arctic drilling read between the lines and declared this a move that would allow that committee's chairwoman, Murkowski, to open ANWR to drilling with a simple 51-senator majority vote.

The fight over drilling in ANWR has raged for decades, mostly at the hands of Alaska's congressional delegation, various members of which have tried a variety of tactics to pass legislation necessary to allow drilling there.

In 1980, the Alaska National Interest Lands Conservation Act, or ANILCA, expanded the refuge, with a compromise: Section 1002 of the law set aside a 1.5 million-acre coastal area for study and potential oil and gas drilling. But anything more than early testing would require congressional approval and then the signature of the president.

The refuge includes 19 million acres of land, about the size of the state of South Carolina, Murkowski said Thursday. Of that, 8 million acres are designated as federal wilderness. The 1002 area includes 1.5 million acres, which Murkowski noted is a little bigger than the state of Delaware.

The instruction to find $1 billion in revenue is "pretty wide open," Murkowski said Thursday afternoon in a speech on the Senate floor, perhaps offering a reason for her Republican colleagues who have previously opposed drilling in ANWR to vote down the amendment.

"Yes – opening the non-wilderness 1002 area to development is an option to meet the instructions to the energy committee. But it is not the only option. But I will tell you it is the best option. And it is on the table," Murkowski said.

"But we should be clear," she said: The amendment offered by Sen. Maria Cantwell, D-Wash., was not a vote on whether to keep ANWR open or closed to drilling. It would simply remove the dictate that the committee find options for new federal revenue. Cantwell is the top Democrat on Murkowski's energy committee.

Republicans who have long opposed drilling in the refuge include Arizona Sen. John McCain and Maine Sen. Susan Collins — both of whom have said they are open to new arguments about the safety of drilling on the refuge's coastal plain.

McCain was joined by Alaska's senators to vote against Cantwell's amendment Thursday night. Collins was not convinced. (West Virginia Democrat Sen. Joe Manchin joined Republicans to provide an additional vote against the amendment.)

"The Arctic National Wildlife Refuge is one of the most pristine areas of the United States and we have been protecting it for decades for a reason," Cantwell said on the Senate floor before the vote late Thursday night.

Drilling there would risk the well-being of caribou, polar bears and 200 species of migratory birds, she said. "For what, oil that we don't need?"

The Interior secretary is already angling to open a billion acres of federal land. ANWR is not a useful addition, she argued.

To make her provision work, Murkowski will have to make the math work — showing that selling leases in ANWR, either alone or with some other provisions, will raise $1 billion over the next 10 years.

Oil prices remain low, and drilling in Alaska is more costly than in some other places. Questions remain as to just how lucrative it would be to tap into the reserves on the coastal plain.

The Center for American Progress, or CAP — a left-wing group opposed to drilling in the Arctic — argued recently that selling oil and gas leases in ANWR would fall far short of the $1 billion to $1.8 billion that its supporters say it will raise. CAP, by contrast, estimated it would only add $37.5 million into the Treasury's coffers.

Murkowski will also have to sell just about all of her Republican colleagues on drilling in ANWR — an effort on which she and Alaska Sen. Dan Sullivan, a Republican, have already embarked.

The pair have been pushing a new argument in recent weeks, painting their position as one born of new and innovative technology, and opponents as relics of the 1980s. The footprint of any new Arctic drilling, they said, will be far smaller than it would have been in the past.

Drilling in ANWR would require just 2,000 acres of surface production, Murkowski said on the Senate floor Thursday. Since drilling began in nearby Prudhoe Bay 40 years ago, "well pads have shrunk by over 80 percent," and subsurface reach now expands miles in each direction, Murkowski said.

Sullivan spoke of limits on Alaska exploration — only in the winter, only with ice roads and on ice pads, meant to have zero impact on the Arctic tundra.

Murkowski argued that the successful development of Prudhoe Bay also showed that "resource development and ecological development can coexist."

In the 1970s, environmentalists argued that building the 800-mile Trans Alaska Pipeline System, or TAPS, would decimate the Central Arctic Caribou herd, Sullivan said. There were 5,000 in the herd then, he said. Today, there are 66,000. "I don't think the herd was decimated."

The "talking points" of "a few experts on Alaska from states like Massachusetts, Oregon" are "just stale," Sullivan said on the Senate floor Thursday morning. Sullivan said he took issue with "senators coming down from places like Massachusetts, Oregon, Washington, Vermont, Rhode Island, talking about Alaska's environment."

"First: Senator Murkowski and I, we care a lot more about the environment, the wildlife, the pristine wilderness in our great amazing state than any other member of this body," Sullivan said.

"Don't believe these doomsday scenarios. Don't believe the misinformed commentary," he said.

Sen. Edward Markey, D-Mass., a longtime opponent of drilling in ANWR, called it a "big oil polar payout" in a press conference on the lawn of the Capitol Building on Tuesday.

Since the passage of ANILCA in 1980, Alaska's congressional delegation — usually all Republicans, but not always — has uniformly sought legislation to allow drilling in the refuge.

They got close in 1989, but efforts were derailed after the disastrous Exxon Valdez oil spill. Congress approved exploration in the refuge in a 1995 bill, but it was vetoed by President Bill Clinton. The delegation got close to including it in must-pass budget and defense bills in 2005.

Now the delegation faces what some proponents call its best chance yet, the fiscal year 2018 budget reconciliation, a bill that the Senate and House Republicans, and President Donald Trump, hope to use to overhaul the U.S. tax system. By using budget reconciliation, they will only need 51 votes to pass (or 50 and a tie-breaker from Vice President Mike Pence). That's a helpful threshold, with only 52 Republicans holding the majority in the Senate.