The broader they are, the more likely liberals are to oppose them, and oppose expanding them. Replacing the progressive income tax system with something regressive isn't something that's going to get support on the left.
> Gas tax? Hugely regressive.
But very narrow.
> Wouldn't liberals support additional carbon taxes?
Many liberals support the concept of carbon taxes as a behavior control mechanism, but not as the main way of funding government, and many raise concerns about the regressive impact. But, with them, the goal is to limit the thing being taxed and fund efforts to improve our ability to avoid it, not be the primary funding method for government.
> Value-added tax?
Virtually all the support I've seen on this in the US is on the right (often competing against flat tax proposals.)
> What about the tax of inflation that serves to redistribute wealth from savers to debtors? (middle class -> government)
While the classical middle class (e.g., Marx's petit bourgeoisie) are net holders of assets, the "middle class" as the term is used in modern discussions in the US (which is largely the middle income segment of wage laborers) aren't really net savers.
