This Trump endorsement is considered so big (relatively) that it made the front page of the FOX News website! The last minute endorsement comes from the son of Roberto Clemente. You might wonder how valuable the endorsement of a sports celebrity's son is. There is not much data or anecdotal evidence on that, but even real sports celebrities' endorsements don't seem to make a big difference.
As far as celebrities go, they didn't come much bigger than Babe Ruth and Lou Gehrig who endorsed presidential candidate Al Smith in 1928. Spoiler alert, Smith lost.
If celebrity endorsements are worth anything, Kamala will have to be satisfied with Lebron James, Stephen Curry, Steve Kerr, Magic Johnson, Harrison Ford, Taylor Swift, Beyonce, Smokey Robinson, Willie Nelson, George Clooney, Julia Roberts, Oprah, Jennifer Lopez, etc., etc...