A dozen House Republicans, half of whom voted for the House tax overhaul bill that passed Nov. 13, wrote a letter to GOP leaders urging them not to allow oil drilling in the Arctic National Wildlife Refuge, adding another complication to negotiating a tax bill that can pass both chambers.

The Senate tax overhaul bill is tied in a reconciliation measure with legislation that would open up drilling parts of the ANWR. Its inclusion is seen as key to having secured GOP Alaska Sen. Lisa Murkowski’s support for the measure.

A group of 12 House Republicans sent a letter Thursday to Senate Majority Leader Mitch McConnell of Kentucky saying they have concerns about any proposal that would implement an ANWR oil and gas leasing program.

“For decades, Congress has voted to prohibit oil and gas development in the Refuge, with the overwhelming support of the American public,” the group wrote. “Support for this protection remains strong today. After years of debate, the Arctic Refuge stands as a symbol of our nation’s strong and enduring natural legacy.”

The House tax bill did not include an ANWR component. The members who signed the letter include six who voted for the House tax bill: Reps. Brian Fitzpatrick, Ryan A. Costello and Patrick Meehan of Pennsylvania, Dave Reichert of Washington, Mark Sanford of South Carolina and Carlos Curbelo of Florida.

It was not clear from the letter whether their concerns would be enough to prevent them from voting for a final tax bill if it included the ANWR drilling proposal. Reichert, Meehan and Curbelo are all members of the Ways and Means Committee. Most of the House conferees on the tax bill will likely be tax writers.

The letter was also signed by six members who voted against the tax bill: Reps. Frank A. LoBiondo, Christopher H. Smith and Leonard Lance of New Jersey and Dan Donovan, Elise Stefanik and John J. Faso of New York. They opposed the tax measure because of concerns about the partial elimination of the state and local tax deduction.

All of the signers except for Sanford and Smith are members of the moderate GOP Tuesday Group.

Sanford is a member of the conservative House Freedom Caucus. Smith is not affiliated of either of the three major GOP caucuses.