It's simple...there are Republicans who have NEVER voted for a Democrat, but who are appalled by what they see in this campaign and by the abuses of the last 8 years. They want to vote to keep any sort of Republican influence that is possible, but are torn by an overwhelming frustration with their choices.
What they will actually do may not be clear until they face the ballot.
There are also a few 'independents' who are just the dithering sort and kid themselves that they have any real idea what they actually think....these are not a huge sample, but may decide based on some intangible whim in the last hours.