I think really since the Tea Party rise, the Republicans have been MUCH better about presenting a single unified message.
Obama was a good enough politician that he overcame it, but in general the Democrats have been a mess. One time they won by saying "we get you hate Trump, so give us a sec to figure it out", and then they were like "oh, so run Biden again, and have everyone campaign on not being Trump!"
I'm actually hoping this starts a backlash around running negative, when a whole party ran on a platform that was literally "not him".
I would go back at least as far as Newt Gingrich's "Contract with America," and maybe further than that. As long as I can remember--and my political memories go back to the mid-to-late 80s--started voting in 1992--Democrats have complained internally about a lack of message discipline / unity as compared to the Republicans. I think part of the problem is the nature of "liberal" vs. "conservative." Liberal, by definition, seeks change; conservative seeks maintenance of the status quo (I'm painting with a broad brush, and, to be honest, I'm not sure that "conservative" is an accurate definition of the current state of the Republican party--but I'm a Democrat, so my opinion on that is highly suspect). It's naturally easier to coalesce around a message of don't screw things up, as opposed to agreeing on what kind of change is the right kind of change. The flip side of that is that people generally do want change, it's just a question of what kind of change.
Negative Campaigning:
Negative campaigning is very effective, that's why it's used. Its primary use is to dissuade voters from a candidate they haven't already picked, and it works. However, it only works if the voter is disconnected from the candidate,
and the message is one that resonates with the audience. Local races are rarely about national issues, so your negative campaign needs to be about something like allowing a porn shop next to a kindergarten, or allowing those evil people in Columbus (pick your state capitol) to take away your local control. That's hard to pull off effectively. If I know candidate x, even just from running into her at the grocery store, it's going to be harder for me to believe that she's advocating for the porn shop next to the school. Surprisingly, even at the local level, there are plenty of people who don't know the candidates well and are susceptible to the negative campaigning. But if a candidate really leans into it, they need to be prepared for the blowback; there will be a lot of voters who do know the other person quite well, and see the negative campaigning as unseemly and tell their friends such. That can be pretty damaging for the candidate who runs the ads. As a result, in my area, PACs do most of it, and there isn't a lot at the super local level (city council districts), but there is at the county level (we call them County Supervisors, but I know they have different names in different places).
I think that is a long way of saying that I agree with Medina about negative campaigning's effectiveness.
Also, one person's "negative campaign" is another person's "running on the issues." I don't see any way to curtail it that can survive the 1st Amendment. I can live with that.