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Like I said, not yet fleshed out :) Maybe others can chime in to explore if this can be used as an attack

In reality, such an attack would not be likely to be carried out by a government or other large group because they already have enough money to just buy their way into the top 20 anyway ;)
However, the idea of downvoting accounts while also being able to upvote accounts would certainly be a less noticeable way to takeover the system, since the downvoting and witness account running could be spread over numerous accounts, making it hard to track and co-relate the actions to one another.

I do think there is some merit to reducing the number of witness votes though, maybe 9 is a good idea, so that the same SP cannot be used to affect more than 50% of the witnesses.