Many Republicans and Democrats would like to bring the $2.3 trillion in untaxed corporate foreign earnings that are parked overseas back to the United States and tax it to make up a massive gap in financing for highway and bridge construction.

But whether such a repatriation would actually raise money for the Treasury or turn into a massive tax loser is a complicated question involving details about timing, the specifics of the policy and what comes after.

Lawmakers are considering several scenarios.

But first, some background: The United States taxes companies' foreign income at the 35 percent corporate tax rate, highest among advanced economies, allowing credits for taxes paid to foreign governments.

Companies can avoid the tax by deferring repatriation of profits earned overseas and reinvesting them abroad. That dynamic has led to the accumulation of the $2.3 trillion in overseas earnings, as estimated by the research firm Audit Analytics.

Meanwhile, the highway trust fund, normally filled by gasoline tax revenue, will run out by the end of July. Republicans have ruled out raising the gas tax, so lawmakers must find $175 billion to meet the shortfall over 10 years.

Here are the proposals for how that could be done by taxing deferred overseas earnings:

A tax holiday

Earlier this year, Sens. Rand Paul, R-Ky., and Barbara Boxer, D-Calif., proposed allowing a repatriation "holiday."

With the holiday, companies could choose to bring back profits into the U.S. over two years and pay a special low rate of 6.5 percent in taxes on the earnings.

Such a plan is favored by many on the Right who like low taxes. But the Joint Committee on Taxation, Congress' nonpartisan tax experts, estimated that the legislation would lose the Treasury $118 billion over a decade.

The policy would raise revenue over the first few years, as companies took advantage of the lower rate to bring back funds.

But the measure would reduce taxes overall, partly because some companies would receive a break on profits they would have repatriated anyway and partly because in later years executives would hoard revenues overseas in anticipation of another holiday.

That appears to be what happened following the repatriation of 2004. A much-cited 2011 report from Senate Democrats found that the holiday cost the Treasury $3.3 billion, while the companies that benefited most did not expand their operations or add workers as was hoped.

Following that experience, Democrats have decried the idea of a repatriation holiday as a giveaway that cannot raise revenues. In a late June hearing on the highway trust fund, Texas Democrat Rep. Lloyd Doggett described the measure as "fairy dust."

Obama's repatriation plan

However, there is a way to raise revenue by taxing deferred earnings, which is to tie the tax to the imposition of a new tax regime.

President Obama included such a proposal in his fiscal 2016 tax plan. The government would enact a mandatory 14 percent tax on all deferred earnings — a maneuver known as a "deemed repatriation" because the earnings are being taxed even if they're not brought back — as part of creating a new international tax regime in which U.S. companies overseas earnings are taxed at 19 percent, rather than the current 35 percent rate.

The deemed repatriation would raise $268 billion over 10 years, the Joint Committee on Taxation projected, enough to plug the gap in the highway trust fund and more.

The one-time special low tax rate raises money because companies wouldn't be able to plan to wait for future holidays or otherwise game the system. They would immediately face the new tax regime once the one-time 14 percent tax was collected.

Such a maneuver, however, would be hard to sell to Republicans fearful of tax hikes.

Broader tax reform

Instead, Republicans would prefer such a move as part of a broader tax reform.

Republican House Ways and Means Committee Chairman Paul Ryan set the stakes high earlier in the year for any deal to use repatriation to fund infrastructure, saying that the "only way repatriation can actually work to help with the highway trust fund problem is through comprehensive tax reform."

More recently, Ryan's counterpart in the Senate, Finance Committee Chairman Orrin Hatch, downplayed the possibility of repatriation altogether, telling the Washington Post last week that "we need to fix the highway process so that it's done with monies that we get from [eliminating] programs that aren't really that effective and should not be in existence."

While Republicans might be resistant to a deemed repatriation, the ongoing problems faced by corporations provide an incentive to get a deal done. Congressional Republicans say they're working on proposals to fix the problems that are forcing U.S. multinationals to keep earnings offshore and even leave the country through mergers or buyouts.

Those are the same dynamics that led former GOP Ways and Means Chairman Dave Camp to include a deemed repatriation proposal in his tax reform discussion draft released last year. Camp would have included a deemed repatriation tax while moving to a new territorial system exempting 95 of all foreign income, raising $170 billion over 10 years.

"Current repatriation proposals are not that simple nor are they without serious policy implications," Rep. Dave Reichert, R-Wash., head of the subcommittee on taxes, said at a late June hearing on repatriation proposals.

"What we know to be true is that repatriation cannot be done as a standalone," Reichert said. "It must be part of a transition to a more competitive system."