Republican efforts to open the Arctic National Wildlife Refuge coastal plain to drilling could soon pass after decades of failed attempts. But if approval comes, oil likely won't flow for a decade.

The measure to allow leasing in the refuge was written by Alaska Sen. Lisa Murkowski and is part of the Senate's tax-overhaul bill.

Republican negotiators with the House and Senate, meeting Wednesday to resolve differences in the tax-overhaul bills that have passed both chambers, agreed to include the ANWR language in a final bill that will go before both houses for a vote, said Rep. Don Young on Thursday, one of the negotiators along with Murkowski.

Murkowski said last week she is cautious about her measure's chances. Congressional efforts to pass the bill and allow drilling in the 19-million-acre refuge in northeast Alaska have nearly won approval in the past, only to die, she said.

Most notably, President Bill Clinton vetoed a 1995 measure that cleared both chambers. Republican President Donald Trump has made clear he supports opening the refuge.

"I think our odds are certainly better this time around," Murkowski said.

If her measure is enacted, the oil, and lucrative royalty income, likely won't flow for about 10 years, she said. That's a typical North Slope timeline as environmental permitting standards are met and remote Arctic conditions challenge plans.

Opponents of the measure say drilling in ANWR could face extra challenges, in part because conservation groups might file suit at every opportunity. They say the opposition will discourage interest from companies already worried about low oil prices and the costs of exploring and developing the remote field.

Murkowski's measure calls on the Bureau of Land Management to hold two lease sales in ANWR within seven years after the bill's enactment. The first lease sale must be held within four years.

Environmental groups have argued the measure will threaten caribou that calve in the 1.5-million-acre section of coastal plain where development might occur, and endanger polar bears that increasingly spend time on land because sea ice is melting.

Kristen Monsell, oceans program legal director at the Center for Biological Diversity, said there will likely be opportunities for legal challenges by conservation groups as federal agencies conduct environmental reviews before leasing is permitted.

"The Trump administration has issued directives to expedite the environmental review process, so I think as a result of that we'll see environmental analyses that are woefully inadequate," she said.

She said one opportunity could come under the Endangered Species Act, designed to protect animals such as the polar bear, classified as threatened.

"Arguably, we could foresee where we could have a claim that drilling in the Arctic does jeopardize polar bears," she said.

Drilling in the coastal plain would directly destroy polar bear habitat, she said. Indirectly, it would melt the bear's sea ice habitat, as new oil production and use boosts greenhouse gas emissions.

Murkowski said leasing in ANWR will be subject to all environmental laws. She has stressed on the Senate floor that "we in no way erode any of the environmental protections."

Environmental reviews will take into account the Clean Air Act, the Clean Water Act, the Endangered Species Act, and the National Environmental Policy Act, she said.

With legal challenges and other unknowns, it could take years to unlock ANWR to drilling, setting up the prospect of long-term political risks, said Larry Persily, a former federal pipeline official in the Obama administration.

A new administration and Congress in 2021 could take steps to stop development, he said. "Alaskans shouldn't start counting the barrels yet," Persily said.

Matt Lee-Ashley, a senior fellow at the Center for American Progress, a liberal think tank, said the coastal plain is the refuge's biological heart. It hosts two-thirds of the refuge's polar bear denning habitat, he said, pointing to a newly released study funded by the center.

"ANWR would be a high-profile area subject to a lot of controversy," Lee-Ashley said. "It's likely that delays with litigation or policy would cause (industry) investments to not bear fruit."

Monsell said a first step after enactment would likely be a "call for nominations" from the public, so BLM knows what acreage industry might be interested in leasing. The legislation requires the agency to offer at least 400,000 acres in each lease sale, covering areas that most likely host oil and gas.

Soon after, the agency would take public input as it develops environmental statements, before finalizing a sale, she said.

In Alaska, that's typically a long process. How things will play out this time is uncertain, Monsell said.

"This is an administration that likes to cut all sorts of corners, so this could be an entirely different effort than what we're used to, to give the oil industry what they want," she said.

BLM is waiting for the measure to pass before taking next steps, said Lesli Ellis-Wouters, the agency's communications director in Alaska.