Thanks to obamatax they now have precedent to move this forward.
From the second link
The Obama administration wants to spend $556 billion over the next six years on transportation projects, but they may have to get creative to pay for them. One "practical option" could be taxing drivers based on their mileage, the nonpartisan Congressional Budget Office (CBO) is reporting.
The CBO has released a report on a potential mileage tax at the behest of Democratic Sen. Kent Conrad (N.D.), the chairman of the Senate Budget Committee, the Hill reports. While tracking mileage was once considered infeasible, the CBO concluded that new technologies
Congress already had the ability to tax in whichever way they chose fit.
They did not need a "precedent" to levy a tax.
The tax was rejected because it was stupid, not because it was unconstitutional.
Yes, BigReb, sometimes the President actually agrees with you.