I believe OP is suggesting that the next election will see domestic individuals or cohorts of them with the wherewithal and motivation to at least significantly replicate the reach of a government propaganda machine. Plus, digital advertising's reach has grown cheaper and broader over time, why wouldn't digital propaganda follow the same pattern?

I think there's room for debate on what draws the line between "propaganda subverting the election" and "leveraging our modern scary-effective digital marketing techniques to spread your electoral message." Does it only count as an attack on democracy if a foreign state or individual is doing it?

Both parties made use of shops like Correct the Record in the last election, what does it mean when in addition to the campaigns, wealthy and influential individuals can exert even more reach in the next election? Is it only propaganda if the message is factually false or misrepresented, or is it enough to have enough money to spend or leverage over the platform that you crowd out all alternate dialogue? Is there a definable ethical spectrum for subversive digital messaging? I think there's plenty of subversive messaging online already, that's advertising. Personally, I think creating fake Americans on twitter to influence the American election crossed whatever the line is. It fails the smell test, it stinks of deception.