The frequency with which vehicle miles tax (VMT) pops up is insane. It is a very, very bad idea (in our current situation, and maybe in any situation). A higher gas tax in superior in almost every way imaginable...
1) We already have a gas tax, so increasing it incurs ~$0 in terms of collection costs.
2) There is no "government tracking" issue to deal with.
3) There is nearly zero fraud potential to deal with.
4) It appropriately encourages people to shift to higher mileage vehicles which:
a) provides additional benefits in terms of lower carbon emissions and
b) are generally lighter and do less damage to the roads than heavier vehicles.
5) Congestion pricing is built in, albeit very small by comparison.
There are maybe a couple downsides to a gas tax over a VMT. One is that that congestion pricing is not really very good, but the VMT would almost have to be the [much more unpopular] GPS type to account for this. The other really doesn't come up until we live in a more utopic society: basically if a very large fraction of cars don't use gas, then there will be less revenue from the gas tax, but we will still have road damage. While I see this "problem" as a huge benefit, and one I think we should look forward to having to deal with, it is a long, long, long way off (and when it ever arrives some form of weight-adjusted VMT may be an appropriate solution).
Oh, and as for increasing the gas tax being politically difficult: I can't even begin to comprehend how it is that people think that the VMT will be [comparatively] popular.