Obama administration officials and key members of Congress want to tax overseas corporate profits to fund highway construction, but they face one major obstacle: Making sure the move isn't a tax increase that conservatives would reject.

Doing that will require expert legislative drafting, preparing a bill that would raise money while simultaneously appearing not to be a tax cut nor a tax hike.

Rep. Paul Ryan, chairman of the House Ways and Means Committee, said Friday that scorekeeping, or working to obtain a favorable cost estimate for the legislation, was the biggest consideration preventing him from rolling out a bill immediately. "Drafting, and scorekeeping, and negotiating," Ryan told reporters in the Capitol, summing up the current behind-the-scenes efforts.

Ryan favors a six-year bill that would bring in a "one-time shot" of about $100 billion in revenue through what is known as "deemed repatriation." Instead of waiting for companies to bring the estimated $2.3 trillion in earnings they have overseas, the government would tax them at a special low rate well below the 35 percent statutory rate — essentially "deeming" that they had been repatriated even though they weren't. That can only be done as part of tax reform, in Ryan's view.

One possible such reform is the framework offered by Sens. Rob Portman, R-Ohio, and Chuck Schumer, D-N.Y., which essentially would stop taxing U.S. businesses on foreign income in the future.

Treasury Secretary Jack Lew expressed support for that approach Friday, saying in New York that "there may not be complete overlap in where we start out in all the details, but that's the conversation."

Apart from other questions about the specifics of the reform proposal, whether or not it would be regarded as raising taxes is a major question for Ryan, whose Republican caucus is against tax hikes.

Grover Norquist, head of Americans for Tax Reform, issued a preemptive warning about the emerging negotiations on Thursday, writing on Twitter that "deemed repatriation is a tax hike. It must be fully offset with tax cuts. Using it instead to pay for highway spending is a Pledge violation." The Pledge, which is maintained by the group and has been signed by most Republicans, is a promise by lawmakers not to raise taxes.

To not be considered a tax increase, the deemed repatriation "must be offset by corresponding tax relief," said Ryan Ellis, the group's tax policy director.

"There are any number of ways to offset the tax increase," Ellis wrote in an email to the Washington Examiner, adding that the group was "confident" that was the context for Ryan's remarks about funding highway spending.

Ryan addressed Norquist's comments Friday. "What conservative tax experts will say, and I know Grover very well, will be if it's part of a broader tax reform package that's a good thing," Ryan said of whether the plan could work without entailing a tax increase.

"It won't be a tax increase, I can assure you that," Ryan added.

Nevertheless, "it's going to be tough," said Curtis Dubay, a tax expert at the conservative Heritage Foundation.

Dubay noted that the deemed repatriation would raise revenue, while other main provisions of the Portman-Schumer strategy, in particular the switch to no longer collecting taxes on overseas income, would lose them. Among all the moving parts, it would be necessary to find a way that the final score from the Joint Tax Committee show the plan neither losing billions for the Treasury nor raising taxes, while still freeing up cash to be put in the Highway Trust Fund — a tall order.

Ryan's predecessor on the tax-writing committee, Rep. Dave Camp of Michigan, managed to pull it off in his draft tax reform proposal released last year. That plan would have raised roughly $170 billion through a deemed repatriation and applied $126.5 billion of it to the Highway Trust Fund.

Camp's plan, however, was much larger in scope than the Portman-Schumer outline, touching all aspects of the individual and corporate sides of the codes.

The size and complexity of the bill gave Camp more flexibility in freeing up cash for highways when it was scored by the Joint Committee on Taxation, but it also made it more confusing where those funds were coming from.

Some budget experts, such as those at the Committee for a Responsible Federal Budget, suggested that Camp "double-counted" the highway funds, using them both to lower tax rates and to fund highway spending.

"So until we see the JCT score, I don't think we can know if it's a hike or a cut," Dubay said.