Well I don't believe it is going to happen this year, but if it did it would mean an instant 30% haircut for all MLPs and maybe a drift-down overshoot to 50%. In addition distributions would be cut on average by about a third. There is actually a precedent...The last government action that compromised MLP tax advantages was the "Lakehead Ruling" nearly 15 years ago. The ruling gave MLP unitholders a 25% haircut before it was overturned.
So, while unlikely, if it did come to pass there's no way it wouldn't have a major impact.