Have you ever been in the back office of a typical political campaign?

Yes. I've had many roles including leadership positions in political campaigns, and including positions where I crafted official campaign language. I have many friends who have held state directorships or higher roles in winning presidential (and other) campaigns. However, politics has not been my main livelihood, so I think I'm informed while also avoiding extremes of idealism (and cynicism) on this front.

Slogans are certainly short-cuts (reductive), tendentious and often misleading. They are totally partisan, but they are also informative. It's certainly upset me when one candidate can spend a lot to get their misleading slogan out and we don't have the money to get our counter-slogan out. I regret the crucial role of money generally in U.S. democracy. However, "slogans" are not "money", and I do think sloganeering/ads are positive, not negative (and also not optimal). Also "slogans" are not "factionalism" -- another problem that I think you are right to be interested in and concerned with (but one that is very difficult to analyze). Also "slogans" are not negativism or necessarily fear-based even if it maybe seems like that right now; in '08, both U.S. pres. candidates emphasized mostly positive/uplifting (typically low-info) slogans.

Since the purpose of a slogan is to deceive, the more susceptible voting is to slogans, the worse the outcomes will be.

This is not right, for several reasons.

First, in practical terms: since outright lies (often) get caught, provocative statements that have some grain of truth are the ones that get picked up. These competing slogans are often stupid (very low signal-to-noise ratio), but the noise -- on average -- cancels out, resulting in people on average getting slightly better chances to make the right decision. E.g.: CA doesn't get every proposition right, but through the haze of slogans back and forth on -- say, prop 23 -- people make the right choice far more often than can be attributed to chance. Yet, all but the very, very few most informed voters seem to be voting based on the slogan/ad/heuristic that mysteriously resonated with them the most.

Second, from a logical perspective: in a 2-person election we start from a fair coin-flip. How do slogans make things worse? Do they "crowd out" intelligent analysis, so that what could have been 55% chance to get the right person becomes a 50% chance? Crowding-out presumes fixed amount of communication bandwidth, which seems for all practical purposes false. Or do you imagine that somehow slogans make the election worse than a coin-flip, taking a 50% chance of success down to only a 40% chance? I can't see how you'd reach that conclusion without a very elaborate and speculative theory.

I differ on Germany; that historical pathology developed through the action of intellectuals, artists, economists, and philosophers, as well as through the compliance of apathetic and ignorant people. Slogans played a role along with every other form of communication, but we can't single out that form of communication -- even though some people may know them better than they know Baeumler or Riefenstahl. Additionally, I don't think post-WWII limitations on German political speech were eliminations of jingoes or slogans: what's criminalized is all endorsements, both sloganeering and seriously intellectual, of certain ideas.

Agree with your concern about partisan capture of education; perhaps isolating curriculum development from elected officials mitigates. Culture somewhat helps, though there are few countries with strong, practical free-speech protections.