Pence and other speakers have implied, if not directly stated, that Biden and Harris don't love America -- or at least not as much as Pence and Trump. Pence said that America is "a nation that deserves more gratitude than grievance." Rhetoric from politicians is often intended to obscure differences and the core ideas at play -- and in this case neither party is really being honest about what it is talking about. The entire discussion about racism and racial inequality happening outside of politics has an element of, "Has America always been pretty racist, and it is pretty racist now?"
A lot of non-politicians on the left are saying "yes" to both of those questions, a lot of people on the right think the answer to the second one in particular is "not really." Pence is implying there is too much focus on what is wrong with America -- namely too much focus on racism in America. Biden and Harris are trying to suggest that there is racism in policing in particular, but it can be addressed by somewhat minor reforms -- and I am guessing that argument is going to get harder to sustain with NBA players and others suggesting the policing problems in America are really deep.