I doubt anyone will have influence under this system except the one who has access to the money. All donations will be anonymous so even when the party comes into power, there is no way to reward the biggest donors an Politics is all about having influence through donations.
While I would support something like this, the issue you raised is actually one of the problems. In the current political donation scheme, donors are clearly identified most of the time, unless the donations are really tiny (even then, the names are recorded, it's just easier to put a random name on a 5 dollar political donation, rather than a 20k one). This would actually be a decent use case for a smart contract like Slock.it would have been; you could vote by holding coins. This raises yet another problem. Is it OK to allow the biggest donor to have the most influence over.the voting process? And what if a rival party corners the market on the coin, to influence votes?