They're not political, or at least nor fundamentally political.
That's like saying that firefighters are political. The ACLU protects everyone's civil liberties, regardless of political affiliation. Government repression runs across the board, its only that Republicans (and I say this only because a Republican is currently in power, it was the same with the Democrats back in Clinton's day) freely give up their rights because they believe that partiotism is blindly following the government and being obedient.
Now, its not exactly like this in reality, but I think that the majority of what they fight for is not specific to one party or idealogy. Sure, they may do some things which could be considerd liberal, but the bulk of their work goes to protect all citizens.