I believe a relevant policy page is here, basically unless you're a charity you can't use the donate button or they can freeze your account. The buy now button is available for others but is only supposed to be used for physical goods apparently, not sure whether intangibles like subscriptions or software qualify. I know some places just use the buy now button and sell crummy little tokens or somesuch and people basically make donations that way. Still, political parties should definitely be eligible for the donate button and it seems like an oversight on google's part that they're not.
In any case google's policies with their ad services are pretty dreadful. I know several (by which I mean at least 2 I can recall) sites/people that had their accounts/funds frozen after their site was linked by a big site (slashdot, reddit, digg, etc) or made the news. The sudden big spike in traffic was deemed suspicious since such a spike in traffic clearly could only be the result of trying to defraud google. Both cases the people just ditched google for ads because they couldn't get their accounts unfrozen (or at least not easily enough that they gave up first), got a different ad provider and considered the money that was in the account a loss.