There are people in the Republican Party who hate Democrats, but also hate trump.
For instance, a voter who strongly disagrees with abortion being legal in any form, will not support Harris, but at the same time should not support the only president we have ever had that tried to force women to have abortions(trump). Such a voter has three possible choices, vote for trump who is evil, vote for Harris, who they do not like, or vote for no one. There is no good choice, so the voter will put off that decision as much as possible.