It's kind of ridiculous to me that an incumbent President running for reelection is focusing not on his term of office at all but instead his opponent that was President preceding him, but that seems to be the entire gameplan.
The incumbent President and his party (among many others) believe in their bones that his opponent and predecessor is an existential threat to American democracy who shouldn't even be eligible to run again. After all, he interrupted the peaceful transfer of power in an attempted self-coup, culminating in his incitement of violence by his supporters on members of Congress and his own Vice President. Of course that would be their focus!
Oh, and it goes without saying that this is personal for the incumbent President.