I think a tax on gasoline is far easier to implement than a tax on mileage, and makes a lot of sense. The government wants to give incentive to high mileage vehicles and electric vehicles, so unless you have a different rate category for the mileage tax it would effectively punish them.
Pardon? Every vehicle has a weight class, and it gets taxed by mileage based on the weight class; heavier vehicles impose more wear on the roadways. This has been successfully used for long-haul trucks for decades. If the taxes on gasoline are, say, $0.40 a gallon, and the car gets 20mpg, the owner is paying $0.02/mile into transportation funds. A pure-electric car is paying nothing. If they replace their 20mpg car with one that gets 40mpb, they're paying $0.01/mile into transportation funds. If the gasoline tax is raised to $0.60/gal, a 20mpg car is paying $0.03/mile into transportation funds, a 40mpg car is paying $0.015/mile into transportation funds... while the owner of the electric car is still paying nothing. If they're all driving 10,000 miles per year, the 20mpg car pays $300/year in taxes, the 40mpg car is paying $150/year in taxes, and the electric car is paying... nothing. For the same amount of use of public roads.
Now, you can say that this just encourages people to move to higher-mileage cars... and it will -- but in the process of doing so, it will also reduce the tax revenues taken in. Every person converting to an electric car effectively stops supporting road maintenance, because they're no longer buying gasoline, and therefore not paying the gasoline tax. This increases the problem that Corker and Murphy are attempting to solve by increasing the gasoline tax rate. Every person who replaces their car with one that gets better mileage, or that doesn't use gasoline, reduces the tax base -- and increasing the tax rate just makes it more attractive to get out from under it. In order to make up for the reduced revenue, they'll need to raise the tax again, and the cycle repeats itself, while anyone with an electric car is essentially freeloading their street use on the backs of everyone not driving an electric vehicle. Mileage-based taxes are more fairly distributed; you are paying on the basis of the amount of use you make of the roads, and it doesn't matter if you're powering your car with gasoline, propane, electricity, water gas, hydrogen, or happy thoughts.