So, your premise falls apart almost immediately. Who was the big case in 1996, 2000, 2004, 2008, 2016?
Even the ones you list don't work that well. Rodney King was beaten senseless in 1991. The only reason it became an issue in 1992 is that an all-white jury in Simi Valley voted to acquit the cops of all charges. And it was barely an issue in the election, as Bush's failure on the economy even had conservatives defecting in droves.
2012, Trayvon Martin wasn't killed by a cop. While he was the catalyst for the BLM movement, his murder was by a hyped-up vigilante. It really wasn't a big issue in the election, either.
As for George Floyd, he wasn't the only high-profile case of brutality by cops in 2020, was he? There were also Breonna Taylor, Jacob Blake, and Rayshard Brooks. It was almost like the cops couldn't help themselves that year. I would say the BLM demonstrations probably HELPED Trump, because nothing brings out the Republican vote like white people soiling their panties. If Trump had been judged just on his failures on Covid and the economy, he probably would have lost even bigger to Biden.
Except Clinton didn't need the black vote in 1992. He actually WON a plurality among white people because Bush and Perot split the white vote.
If a case of police brutality becomes a big deal this year, it will because Trump is a racist. You know damned well that Trump will hit the imaginary crime issue (even though Crime spiked on his watch in 2020, and has been on the decline since.)