At a corporate taxation conference held in Washington at the end of March, a tax partner from Ernst & Young asked an Internal Revenue Service lawyer in attendance about “rumors” that the IRS had stopped issuing private letter rulings for MLPs, according to an eyewitness account.

Such rulings had been issued with increasing frequency by the IRS to bless new partnerships engaged in drilling, refining, gas compression, wastewater disposal and even the mining of sand used in hydraulic fracking.

Yes, there had been a pause, the IRS lawyer allowed. The agency appears to have become uncomfortable with the proliferation of unconventional MLPs well beyond the traditional scope of the pipeline partnerships, and had paused issuing case-by-case rulings to hold a review and consult with the Treasury.

Asked  if the IRS might backtrack on favorable prior rulings, the IRS lawyer said that night that it might, but described that option as unlikely. The agency subsequently told the MLP industry group that the pause in rulings could last up to three months. Which means it should be completed any day now.

This review is not a threat to the vast majority of MLPs that unequivocally qualify for their tax-advantaged status. Under the terms of the 1987 Revenue Act, this would include any partnership engaged in “the exploration, development, mining or production, processing, refining, transportation (including pipelines transporting gas, oil, or products thereof), or the marketing of any mineral or natural resource (including fertilizer, geothermal energy, or timber).”

But fracking sand and wastewater are nowhere to be found on that list, and if the IRS did want to change its previously liberal interpretation of the rules in favor of new partnerships, those would figure to be the industry groups first in the line of potential fire.

Chances of such revisionism are low but they’re not zero, and nothing the IRS does is being taken for granted these days. I asked one money manager who had been harping on the review’s risks why it should prove any more of a threat than a similar review of real estate investment trusts, which concluded last year without major changes.

There’s no reason to be worried “if you believe political pressure and the actions of the US Treasury and IRS are linear and always predictable,” he responded. Point taken.

Could the IRS crack down on publicly traded partnerships running gas stations? After all, the conference committee report accompanying the 1987 law specifically excluded these. This again seems unlikely, but were it to happen it would certainly be a big blow to Energy Transfer Equity (NYSE: ETE) and Energy Transfer Partners (NYSE: ETP), which have just paid a big premium to bring hundreds of additional gas stations under their tax-free canopy.

But it’s easy to fall into the trap of imagining what the IRS could do. In practice, the agency is likely to find its freedom of maneuver constrained not merely by the letter of the law but also by precedent and political realities. The energy industry has many friends on Capitol Hill, and has gained extra leverage from the success of shale drilling and its contributions to economic growth.

MLP investors are understandably nervous about their tax perks, recalling the “Halloween Massacre” of 2006, when the Canadian government abolished similar tax advantages enjoyed by Canadian trusts in a surprise move. The good news (for MLPs, at least) is that the presidential system of government in the US makes a similar move by the executive impossible, and any changes in Congress seem highly improbable over the medium term given the gridlock paralyzing that institution.