There's also the fact that political propaganda comes in a lot more veiled forms than corporate propaganda. Because it's in a sense natural for us to discuss politics face to face, or at least we recognize that some level of discussion about politics is necessary and good, we're mostly ok with things like celebrities endorsing a candidate. A candidate traveling around the country and speaking to potential voters is pretty much fine. A single person expressing their views
honestly isn't really guilty of "propaganda".
The problem is when huge amounts of money get mixed up in it. In the US money doesn't buy you political power directly, but it does buy you a voice (in the form of advertisements using mass media). It's still up to the listeners to listen to your voice one way or the other, but the disproportionate loudness of people's voices ensures that arguments backed by money are supported much louder than those without money (this is the thesis of Manufactured Consent). Ok, this is less than ideal, but things probably aren't skewed that much regarding things like social issues.
Political advertisement, in my opinion, doesn't veer into the realm of propaganda until one of two things happen: either the source is dishonest about their intentions (e.g. a person fully aware of climate change publicly denying it for financial reasons) and true beliefs, or their arguments are veiled in a way such that they do not appear to be advertisement at all. For example, suppose out of 100 homicides in the US, 10 were committed by Green people against Purple people, but a news organization decides to cover 5 homicides this week, and focuses solely on the ones between green and purple people. That doesn't look like an ad, even though it is one. The problem is that there's big money in this type of propaganda; these days political power is all about controlling narratives. It allows for a type of "inception" of beliefs and values - for example, making Green people think they're on the bring of a race war with Purple people - by letting people come to conclusions themselves after being presented by a highly slanted distribution of input.
This type of belief-inception is precisely what Cambridge Analytica specialized in. By knowing demographic information, they could target individuals based on issues they knew they would be sensitive to, and slowly indoctrinate them with desired views. I'll use my race example again, because Robert Mercer is essentially an unapologetic racist: https://en.wikipedia.org/wiki/Robert_Mercer_(businessman)#Ra.... You start by painting a narrative picture, highlighting race-related conflicts and painting a picture of deteriorating race relations. Like a self-fulfilling prophecy, this stokes racial tensions, creating more incidents for you to curate. By misrepresenting the relative frequency of these types of occurrences, people gradually come to the conclusion that you want them to: in this case, that black people are becoming more racist towards white people. You can use this to bring over working-class white people to the Republicans. Another good example of this type of indoctrination was Gamergate (which essentially birthed Breitbart / the alt-right) being used to galvanize frustration with the social justice sphere into creating a community of young male "race-realists"