>If corrupt incumbent politicians have complete control of the finances of an election, how could they be held accountable?

The exact same way any relief is sought against the government: the court system.

Ok, and who appoints the judges? And who is responsible for enforcing court decisions?

I agree that our current government is not.

But what you are proposing is a different government, one in which leaders could operate without the primary accountability that they face now: that anyone can organize a campaign against them.

The barriers you cite already exist. There are plenty of rules ablout what must do to get on the ballot, yet somehow new politicians still get into politics.