You misunderstand the dynamic. Most GOP voters are going to vote and are going to vote for the Republican, regardless of how awful that Republican is. Voting is a civic duty and party above all are kinda core ideas for them.
Dem voters are a lot more flighty in general. Any barrier to voting no matter how small (even having to rise from the couch) impacts Dem voters more than GOP ones.
There are more Dem voters than GOP ones except maybe in very red states. It’s about turnout - US voter turnout is God awful and it’s worse among Dems than GOP.
That’s why the debate was so bad for the Dems, because it’s not about whether or not it pulls voters to Trump but about what it does to Dem turnout.