https://www.universityofcalifornia.edu/news/why-negative-campaigning-works-and-how-fight-it

Ledgerwood and her colleagues have also found that a negative frame is much more persistent, or “stickier,” than a positive one. If you come at an issue negatively, but are later reminded of the policy’s positive aspects, you will still think it’s a bust. And if you start out thinking favorably about the policy, but are reminded of its downsides, your positive perception will be swept away and a negative one will take its place.
The beauty of negative attacks — from a campaign standpoint — is that they influence everyone. Even a candidate’s supporters will be affected by negative attacks, Ledgerwood and her collaborators have found. Once a negative idea has been planted, it’s very hard to shake.

https://goizueta.emory.edu/research-spotlight/playing-dirty-2020-does-negative-advertising-actually-work-elections

Looking at correlations between the volume of negative ads and the vote shares achieved by U.S. Senate candidates in 2010 and 2012, the researchers found that “while positive political advertising does not affect two-party vote share, negative political advertising has a significant positive effect on two-party vote shares.”

https://www.rochester.edu/newscenter/when-campaign-ads-go-low-it-often-works/

“Negative campaigning has been around as long as campaigning,” Lovett says. “It stays around because it works.”

https://www.cnn.com/2012/01/02/opinion/lariscy-negative-ads/index.html

So if we don’t like negative ads and even perhaps suspect they contribute to political malaise, why are they increasingly dominating candidates’ strategies?
The answer is simple: They work. And they work very well. Gingrich’s drop in polls in Iowa last month was no accident – it was choreographed by negative advertising. . . .
. . . Our brains process information both consciously and non-consciously. When we pay attention to a message we are engaged in active message processing. When we are distracted or not paying attention we may nonetheless passively receive information. There is some evidence that negative messages may be more likely than positive ones to passively register. They “stick” for several reasons.
First, one of the most important contributors to their success may be the negativity bias. Negative information is more memorable than positive – just think how clearly you remember an insult.
Second, negative ads are more complex than positive ones. A positive message that talks about the sponsoring candidate’s voting record, for example, is simple and straightforward. Every negative ad has at least an implied comparison. If Mitt Romney is “not a true conservative,” then by implication the candidate sponsoring the ad is saying he or she is a true conservative. This complexity can cause us to process the information more slowly and with somewhat more attentiveness.