Elections are won at the margins. Even if you convince just a tiny % each time he does something horrible, it can add up. Regardless of minds being changed, it can still affect likelihood of people showing up at the polls

As an aside, I highly recommend volunteering for the campaign. You'll quickly discover that undecideds do indeed exist and some can be certainly be convinced. Quite often they are more low-information voters and unaware of much of republican's horrific plan

Link with places to volunteer around you