Libertarian-leaning Republicans. Democrats are afraid of it because it's unregulated.
This.
I think liberals will also dislike bitcoin because it will force the government to be smaller and less powerful. The liberals generally want government to be as large as possible and for everyone to rely on it.
To be fair, I don't think they want people to rely on government (although the politicians certainly do). They want people to
be able to rely on government, and they're willing to accept the side-effect of people who don't need to rely on the government becoming reliant. For money, they want the government to guarantee the value of their money through the use of force and don't care about the consequences (petrodollar wars, inflation, non-obvious tax burdens on the poor).
I think that a large percentage of people who "can" rely on government will do so, even if they have the ability to support themselves without the help of government. Liberals are well aware of this fact.
Generally speaking the tax burden on the poor is very low. In many cases many poor people's effective total tax rate (including income, sales, property and any other types of taxes) is below zero.