This is a good point that runs a little deeper than some people appreciate. (moving a topic into the category of "political" can already, today, be a subtle form of censorship in the public sphere)

At the same time, if we're talking about advertising, in the US we can just rely on what constitutes political advertising under the law.

I agree that would be an easy move for Google to make.

I would imagine that there would then be an incentive for potential donors to, rather then giving cash, pay for messages that fly under the radar but are designed to support a particular campaign.