This is where ranked choice voting comes in. It's a system where instead of casting a vote for a single candidate, voters rank several choices from first to last. If one candidate gets a majority of first-choice votes, they win. Congratulations, President Vermin Supreme! If no candidate gets a majority of first-choice votes, then the candidate with the fewest first-choice votes is eliminated. Sorry, Senate candidate Ted Nugent, looks like the seat's going to Kid Rock. Votes for the eliminated candidate are redistributed to voters' second choice, then third, then fourth, and so on, until a candidate has a majority.
That means our hypothetical voter up there could feel secure putting their preferred candidate first and then the Democrat second, knowing that if the former gets eliminated, their vote just rolls over to the latter.
While this may not be a perfect system, it's certainly a better system than what we've got. No more being forced to choose between the political equivalent of a sharp stick in the eye and an acid enema. Instead, you could just rank the political equivalent of a basket of handjobs first and put the sharp stick in the eye further down the list.
More candidates could run and be taken seriously, and more voters would thus have a reason to leave the house on Election Day -- something desperately needed in a country where only a little over half of the eligible population bothers to vote (one study associated the practice with a 10 percent increase in voter turnout). Ireland has used ranked choices for presidential elections ever since write-in candidate "Fuck The British" technically became president in 198-I-just-made-that-up. Australia has been using this system for state and federal elections for decades.
Despite some grassroots success in smaller U.S. elections, we'll probably never see it on a federal level. Unless, you know, the big two parties decide they're OK with loosening their stranglehold on the country. That could happen, right?