No you just don't vote for that race. You don't vote for the Republican cuz that just tells the Democrats that you support the Republican cause. That would make them chase Republican ideals even more.

If you find a Democratic candidate so completely abhorrent and you cannot vote for them, then vote for neither. Or vote third party if one of them exists on the ballot and it lines with your values cuz that'll be much more helpful than anything else.

Now normally I wouldn't council any of that mind you, but this does seem like it's pretty dire in this case.